Optical glass, preform for press forming, optical element, and processes for producing these

ABSTRACT

Provided is a low-dispersion optical glass that is formed of a fluorophosphate glass in which the molar ratio of the content of O 2−  to the content of P 5+ , O 2− /P 5+ , is 3.5 or more and that has an Abbe&#39;s number (ν d ) of over 70 or has an F −  content of 65 anionic % or more, and the optical glass enables the suppression of the volatilization of a glass component when an optical glass formed of a fluorophosphate glass is produced or when an obtained glass in a molten state is caused to flow out to shape it into a glass shaped material, so that the variation of properties such as a refractive index, etc., involved in the fluctuations of a glass composition and the variation of quality such as the occurrence of striae can be suppressed.

This application is a divisional of application Ser. No. 12/528,189filed Sep. 3, 2009, now allowed, which in turn is the U.S. nationalphase of International Application No. PCT/JP2008/053897 filed Feb. 27,2008 which designated the U.S. and claims priority to JP Application No.2007-055316 filed Mar. 6, 2007; the entire contents of each of which arehereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a fluorophosphate optical glass andprocess for the production thereof, a press-molding preform and aprocess for the production thereof and an optical element and a processfor the production thereof.

TECHNICAL BACKGROUND

A fluorophosphate optical glass is very useful as a low-dispersionglass, and as such a glass, a glass described in JP10-139451A is known.

DISCLOSURE OF THE INVENTION

When a fluorophosphate glass is produced by heating and melting rawmaterials, a glass component volatilizes from the surface of a glassunder the heated of a high temperature. The amount of a glass obtainedis hence decreased relative to the amount of raw materials used, andproperties such as a refractive index, etc., come to deviate fromintended values. It is therefore required to take some measures such asa little increase in amount when a raw material batch is prepared, withregard to the component that is lost by volatilization. However, suchmeasures can be no fundamental solution for preventing thevolatilization of the above glass components. Further, when a glass isshaped into a glass shaped material by causing an optical glass in amolten state to flow out of a pipe and casting it into a mold, a glasscomponent also volatilizes to cause an optically non-uniform portioncalled striae in a layer in the vicinity of the surface of the glassshaped material.

The present invention has been made in the light of the abovecircumstances, and the object thereof is to provide a low-dispersionoptical glass and a process for the production thereof, which arecapable of suppressing the volatilization of a glass component and thevariation of quality involved in the fluctuations of a glass compositionwhen an optical glass formed of a fluorophosphate glass is produced orwhen the produced glass in a molten state is caused to flow out of apipe and shaped into a glass shaped material.

Further, it is another object of the present invention to provide apress-molding preform formed of the above optical glass and a processfor the production thereof, an optical element blank formed of the aboveglass and a process for the production thereof and an optical elementand a process for the production thereof.

Means to Solve the Problems

The present inventor has made diligent studies, and it has been foundthat the above object can be achieved by an optical glass which isformed of a fluorophosphate glass having an O²⁻ content/P⁵⁺ contentmolar ratio, O²⁻/P⁵⁺, of 3.5 or more and which has an Abbe's number(ν_(d)) of over 70,

an optical glass which is formed of a fluorophosphate glass having anO²⁻ content/P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.5 or more and whichhas an Abbe's number (ν_(d)) of over 78,

an optical glass which is formed of a fluorophosphate glass having anO²⁻ content/P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.5 or more, having atotal rare earth element content of less than 5 cationic % and having anF⁻ content/F⁻ and O²⁻ total content molar ratio, F⁻/(F⁻+O²⁻), of over0.2 and which has a refractive index (n_(d)) of over 1.53 and an Abbe'snumber (ν_(d)) of over 70, and

an optical glass formed of a fluorophosphate glass comprising P⁵⁺ as acationic component and F⁻ and O²⁻ as anionic components and having an F⁻content of 65 anionic % or more and having an O²⁻ content/P⁵⁺ contentmolar ratio, O²⁻/P⁵⁺, of 3.5 or more. On the basis of the above finding,the present invention has been completed.

That is, the present invention provides

(1) an optical glass that has an Abbe's number (ν_(d)) of over 70 andthat is formed of a fluorophosphate glass having an O²⁻ content/P⁵⁺content molar ratio, O²⁻/P⁵⁺, of 3.5 or more (to be referred to as“optical glass I” of the present invention hereinafter),

(2) an optical glass as recited in the above (1), which has an Abbe'snumber (ν_(d)) of over 78 (to be referred to as “optical glass II” ofthe present invention hereinafter),

(3) an optical glass as recited in the above (1), which is formed of afluorophosphate glass having a refractive index (n_(d)) of over 1.53,having a total rare earth element content of less than 5 cationic % andhaving an F⁻ content/F⁻ and O²⁻ total content molar ratio, F⁻/(F⁻+O²⁻),of over 0.2 (to be referred to as “optical glass III” of the presentinvention hereinafter),

(4) an optical glass as recited in the above (1), wherein saidfluorophosphate glass comprises, by cationic %,

3 to 50% of P⁵⁺,

5 to 40% of Al³⁺,

0 to 10% of Mg²⁺,

0 to 30% of Ca²⁺,

0 to 30% of Sr²⁺,

0 to 40% of Ba²⁺,

provided that the total content of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ is 10% ormore,

0 to 30% of Li⁺,

0 to 20% of Na⁺,

0 to 20% of K⁺,

0 to 10% of Y³⁺,

0 to 10% of La³⁺,

0 to 10% of Gd³⁺,

0 to 10% of Yb³⁺,

0 to 10% of B³⁺,

0 to 20% of Zn²⁺ and

0 to 20% of In³⁺,

and comprises, by anionic %,

20 to 95% of F⁻ and

5 to 80% of O²⁻,

(5) an optical glass as recited in the above (2), wherein saidfluorophosphate glass comprises, by cationic %,

3 to 30% of P⁵⁺,

10 to 40% of Al³⁺,

0 to 10% of Mg²⁺,

0 to 30% of Ca²⁺,

0 to 30% of Sr²⁺,

0 to 30% of Ba²⁺,

provided that the total content of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ is 10% ormore,

0 to 30% of Li⁺,

0 to 20% of Na⁺,

0 to 20% of K⁺,

0 to 10% of Y³⁺,

0 to 10% of La³⁺,

0 to 10% of Gd³⁺,

0 to 10% of Yb³⁺,

0 to 10% of B³⁺,

0 to 20% of Zn²⁺ and

0 to 20% of In³⁺,

and comprises, by anionic %,

40 to 95% of F⁻ and

5 to 60% of O²⁻,

(6) an optical glass as recited in the above (3), wherein saidfluorophosphate glass comprises, by cationic %,

20 to 50% of P⁵⁺,

5 to 40% of Al³⁺,

0 to 10% of Mg²⁺,

0 to 20% of Ca²⁺,

0 to 20% of Sr²⁺,

0 to 40% of Ba²⁺,

provided that the total content of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ is 10% ormore,

0 to 30% of Li⁺,

0 to 20% of Na⁺,

0 to 20% of K⁺,

0 to 5% of Y³⁺, exclusive of 5%,

0 to 5% of La³⁺, exclusive of 5%,

0 to 5% of Gd³⁺, exclusive of 5%,

0 to 5% of Yb³⁺, exclusive of 5%,

provided that the total content of Y³⁺, La³⁺, Gd³⁺ and Yb³⁺ is less than5%,

0 to 10% of B³⁺,

0 to 20% of Zn²⁺ and

0 to 20% of In³⁺,

(7) an optical glass as recited in the above (1) or (2), wherein thefluorophosphate glass has an F⁻ content of 65 anionic % or more,

(8) an optical glass formed of a fluorophosphate glass comprising P⁵⁺ asa cationic component and comprising F⁻ and O²⁻ as anionic components,the fluorophosphate glass having an F⁻ content of 65 anionic % or moreand an O²⁻ content/P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.5 or more (tobe referred to as “optical glass IV” of the present inventionhereinafter),

(9) an optical glass as recited in the above (8), which comprises, bycationic %,

3 to 15% of P⁵⁺,

25 to 40% of Al³⁺,

5 to 35% of Ca²⁺, and

5 to 25% of Sr²⁺,

(10) an optical glass as recited in the above (9), which comprises, bycationic %,

0 to 10% of Mg²⁺,

0 to 20% of Ba²⁺,

0 to 20% of Li⁺,

0 to 10% of Na⁺,

0 to 10% of K⁺, and

0 to 5% of Y³⁺,

(11) a process for the production of an optical glass formed of afluorophosphate glass, which comprises using, as raw materials orcullet, a glass composition having a total O²⁻ content/total P⁵⁺ contentmolar ratio, O²⁻/P⁵⁺, of 3.5 or more when an optical glass is producedby melting the raw materials or cullet and refining and homogenizing amolten glass, and thereby producing the optical glass recited in any oneof the above (1) to (10),

(12) a process for the production of an optical glass formed of afluorophosphate glass, which comprises preparing a raw material batchfrom raw materials or cullet, melting the raw material batch and thencarrying out refining and homogenization,

the process comprising preparing the raw material batch in which thetotal O²⁻ content/total P⁵⁺ content molar ratio, O²⁻/P⁵⁺, is 3.5 or moreand carrying out the melting, refining and homogenization to produce afluorophosphate glass having an Abbe's number (ν_(d)) of over 70,

(13) a process for the production of an optical glass formed of afluorophosphate glass, which comprises preparing a raw material batchfrom raw materials or cullet, melting the raw material batch, thencarrying out refining and homogenization to prepare a molten glass andshaping said molten glass,

the process comprising controlling the total O²⁻ content/total P⁵⁺content molar ratio, O²⁻/P⁵⁺, in said raw material batch for decreasingthe volatility of said molten glass,

(14) a process for the production of an optical glass as recited in theabove (13), wherein the fluorophosphate glass having an Abbe's number(ν_(d)) of over 70 is produced,

(15) a process for the production of an optical glass as recited in theabove (12) or (14), wherein the fluorophosphate glass having an Abbe'snumber (ν_(d)) of over 78 is produced,

(16) a process for the production of an optical glass as recited in anyone of the above (12) to (14), wherein the fluorophosphate glass havinga rare earth element total content of less than 5 cationic %, an F⁻content/F⁻ and O²⁻ total content molar ratio, F⁻/(F⁻+O²⁻), of over 0.2and a refractive index (n_(d)) of over 1.53 is produced,

(17) a process for the production of an optical glass as recited in anyone of the above (12) to (15), wherein the fluorophosphate glass havingan F⁻ content of 65 anionic % or more is produced,

(18) a press-molding preform formed of the optical glass recited in anyone of the above (1) to (10) or an optical glass obtained by the processrecited in any one of the above (11) to (17),

(19) a process for the production of a press-molding preform, whichcomprises causing a molten glass to flow out of a pipe to separate amolten glass gob having a predetermined weight and shaping said glassgob into a preform when the glass goes through a process of cooling,

the process comprising shaping the press-molding preform recited in theabove (18),

(20) a process for the production of a press-molding preform, whichcomprises casting a molten glass into a casting mold to prepare a glassshaped material and processing said glass shaped material to make thepress-molding preform,

the process comprising shaping the press-molding preform recited in theabove (18),

(21) an optical element blank for an optical element that is completedby grinding and polishing, which is formed of the optical glass recitedin any one of the above (1) to (10) or an optical glass obtained by theprocess recited in any one of the above (11) to (17),

(22) an optical element formed of the optical glass recited in any oneof the above (1) to (10) or an optical glass obtained by the processrecited in any one of the above (11) to (17),

(23) a process for the production of an optical element blank from whichan optical element is completed by grinding and polishing,

which comprises heating and press-molding the preform recited in theabove (18) or a preform obtained by the process recited in the above(19) or (20),

(24) a process for the production of an optical element blank, whichcomprises melting glass raw materials, causing the resultant moltenglass to flow out, separating a molten glass gob from the molten glassflow and press-molding said molten glass gob,

the process comprising melting and molding the optical glass recited inany one of the above (1) to (10) or an optical glass obtained by theprocess recited in any one of the above (11) to (17),

(25) a process for the production of an optical element, whichcomprising grinding and polishing the optical element blank recited inthe above (21) or an optical element blank produced by the processrecited in the above (23) or (24), and

(26) A process for the production of an optical element, which comprisesheating and precision press-molding the preform recited in the above(18) or a preform obtained by the process recited in the above (19) or(20).

EFFECT OF THE INVENTION

According to the present invention, there can be provided alow-dispersion optical glass and a process for the production thereof,which are capable of suppressing the volatilization of a glass componentand the variation of quality involved in the fluctuations of a glasscomposition when an optical glass formed of a fluorophosphate glass isproduced or when the produced glass in a molten state is caused to flowout of a pipe and shaped into a glass shaped material.

According to the present invention, further, there can be provided apress-molding preform formed of the above optical glass and a processfor the production thereof, an optical element blank formed of the aboveglass and a process for the production thereof and an optical elementand a process for the production thereof.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of a precision press-molding apparatusused in Examples of the present invention.

BEST MODES FOR WORKING THE INVENTION

The present invention suppresses the formation of volatile substancesformed in the step of melting a glass in prior art and decreases thevolatility of a glass to a great extent.

[Optical Glass]

The optical glass of the present invention will be explained below. Indescriptions hereinafter, “%” for a content of a cationic component or atotal content of cationic components stands for “cationic %”, and “%”for a content of an anionic component or a total content of anioniccomponents stands for “anionic %”, unless otherwise specified.

(Optical Glass I)

First, the optical glass I of the present invention will be explained.

The optical glass I of the present invention is an optical glass formedof a fluorophosphate glass having an Abbe's number (ν_(d)) of over 70and having an O²⁻ content/P⁵⁺ content molar ratio of 3.5 or more.

In the fluorophosphate glass having an Abbe's number (ν_(d)) of over 70and having an O² content/P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.5 ormore, for constituting the optical glass I, the O²⁻ content/P⁵⁺ contentmolar ratio, O²⁻/P⁵⁺, is preferably 3.53 or more, more preferably 3.55or more.

In the optical glass I of the present invention, further, the Abbe'snumber (ν_(d)) is preferably over 75, more preferably 78 or more, stillmore preferably 80 or more.

As described already, when a fluorophosphate glass having the propertyof low dispersion as represented by an Abbe's number (ν_(d)) of over 70is produced, glass components volatilize when the glass is produced orwhen a molten glass is caused to flow out. When the present inventor hasmade studies, surprisingly, it has been found that the abovevolatilization can be suppressed by employing a fluorophosphate glasshaving an O²⁻ content/P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.5 or more.

That is, as a raw material for the above fluorophosphate glass,generally, phosphate is used, and for introducing fluorine (F⁻) as ananionic component as much as possible, it is general practice to usemetaphosphate (oxygen atom/phosphorus atom=3) in which the ratio of thenumber of oxygen (O²⁻) atoms per atom of phosphorus (P⁵⁺) is small.

According to studies made by the present inventor, when a glass isproduced from the above metaphosphate as a raw material, metaphosphoricacid derived from the raw material and fluorine react with each other ina molten glass to generate phosphoryl fluoride (POF₃) as a volatilecomponent. In contrast, it has been found that when the atomic ratio ofoxygen atoms per phosphorus atom in the molten glass is adjusted to 3.5or more (oxygen atom/phosphorus atom≧3.5), the generation amount of avolatile component is decreased to a great extent. It is considered thatthe above is because diphosphoric acid in which the ratio of number ofoxygen (O²⁻) atoms per atom of phosphorus (P⁵⁺) (oxygen atom/phosphorusatom) is 3.5 is more stable as a phosphoric acid in a molten glass thanmetaphosphoric acid in which the ratio of number of oxygen (O²⁻) atomsper atom of phosphorus (P⁵⁺) (oxygen atom/phosphorus atom) is 3.

In the optical glass of the present invention, therefore, the molarratio of the content of O²⁻ to the content of P⁵⁺ in a fluorophosphateglass is limited to 3.5 or more and a metaphosphoric-acid-free glass isformed. The generation of phosphoryl fluoride as a volatile component ishence suppressed, and the variation of quality involved in thefluctuations of a glass composition is hence decreased.

The optical glass I preferably includes the following optical glass I-a.

The optical glass I-a is a fluorophosphate glass which comprises, ascationic components,

3 to 50% of P⁵⁺,

5 to 40% of Al³⁺,

0 to 10% of Mg²⁺,

0 to 30% of Ca²⁺,

0 to 30% of Sr²⁺,

0 to 40% of Ba²⁺,

provided that the total content of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ is 10% ormore,

0 to 30% of Li⁺,

0 to 20% of Na⁺,

0 to 20% of K⁺,

0 to 10% of Y³⁺,

0 to 10% of La³⁺,

0 to 10% of Gd³⁺,

0 to 10% of Yb³⁺,

0 to 10% of B³⁺,

0 to 20% of Zn²⁺ and

0 to 20% of In³⁺,

and comprises, as anionic components,

20 to 95% of F⁻ and

5 to 80% of O²⁻.

P⁵⁺ is an essential component that works as a network former in theglass. When the content thereof is less than 3%, the glass is extremelyunstable. When the content thereof exceeds 50%, it is required foradjusting the O²⁻/P⁵⁺ molar ratio to 3.5 or more to limit the content offluorine to be introduced, and the necessary low-dispersion property canbe no longer obtained. The content of P⁵⁺ is therefore preferablyadjusted to the range of 3 to 50%.

Al³⁺ is an essential component for improving the stability of thefluorophosphate glass. When the content thereof is less than 5%, theglass is destabilized. When the content thereof exceeds 40%, the totalcontent of the other components is too small, and the glass isdestabilized. The content of Al³⁺ is therefore preferably adjusted tothe range of 5 to 40%.

Alkaline earth metals such as Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ are componentsthat improve the stability of the glass and increase the refractiveindex. When the total content thereof is adjusted to 10% or more, theeffect thereof on the stability is increased. However, when the contentof a particular alkaline earth metal component is too large, the balancewith other components is broken. Therefore, it is preferred to introducethem evenly, and it is preferred to introduce at least two members ofMg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺. Specifically, preferably, the content of Mg²⁺is limited to 0 to 10%, the content of Ca²⁺ is limited to 0 to 30%, thecontent of Sr²⁺ is limited to 0 to 30% and the content of Ba²⁺ islimited to 0 to 40%.

Alkali metals such as Li⁺, Na⁺ and K⁺ are components that decrease theglass viscosity and the glass transition temperature to expedite theproduction of the glass. When they are introduced to excess, however,they degrade the glass stability. Therefore, preferably, the content ofLi⁺ is limited to 0 to 30%, the content of Na⁺ is limited to 0 to 20%,and the content of K⁺ is limited to 0 to 20%. Of the alkali metals, Li⁺also has a high effect on improvement of the stability. Therefore, it ismore preferred to introduce 0.5% or more of Li⁺, it is still morepreferred to introduce 1% or more of Li⁺, and it is particularlypreferred to introduce 2% or more of Li⁺.

Rare earth elements such as Y³⁺, La³⁺, Gd³⁺, Yb³⁺, etc., are componentsthat increase the refractive index while maintaining the low-dispersionproperty of the glass. When they are introduced to excess, however, theyincrease the melting temperature of the glass and also decrease theglass stability. It is therefore preferred to limit the content of eachof the above components to 0 to 10%.

B³⁺ is a component that improves the durability of the glass. Since,however, it tends to volatilize as a fluoride during melting, it is alsoa component that decreases productivity. Therefore, it is preferred tolimit the content of B³⁺ to 0 to 10%, and it is more preferred to limitthe content thereof to 0 to 5%. It is still more preferred to introduceno B³⁺.

Zn²⁺ and In³⁺ have the property that they can be easily introduced intothe glass like alkaline earth metals, and when Zn²⁺ and In³⁺ areintroduced to form a multi-component glass, it is expected that there isproduced an effect that the stability is improved. However, it isundesirable to introduce them to excess. Therefore, it is preferred tolimit the content of each of Zn²⁺ and In³⁺ to 0 to 20%, it is morepreferred to limit the content of each of Zn²⁺ and In³⁺ to 0 to 10%, andit is still more preferred to limit the content of each of Zn²⁺ and In³⁺to 0 to 5%. It is particularly preferred to introduce none of Zn²⁺ andIn²⁺.

In addition to the low-dispersion property and anomalous partialdispersion property, the optical glass I also has the property ofexhibiting a high light transmittance in a broad region fromshort-wavelengths to long-wavelengths in the visible light region. Whenthese properties are to be utilized, the optical glass I is suitable asa material for obtaining various optical elements such as a lens, aprism and the like. For the above use fields, it is preferred not to addions having absorption in the visible light region, e.g., ions of metalelements such as Fe, Cu, Ni, Co, Cr, Mn, V, Nd, Ho and Er.

On the other hand, the optical glass I can be imparted with the propertyof near infrared absorption by adding Cu²⁺. It is hence desirable to add0.5 to 13% of Cu²⁺ per 100% of the glass composition excluding Cu²⁺. Theglass containing Cu²⁺ is suitable as a material for color collectionfilters for semiconductor image sensing devices such as CCD, CMOS, etc.The amount of Cu²⁺ can be properly determined in the above range whiletaking account of the thickness of the above filters. In the glasscontaining Cu²⁺, it is desirable not to add any other ion havingabsorption in the visible light region than Cu²⁺ unless the absorptioncharacteristic is to be adjusted.

Anion components and anion additives will be explained below. Theoptical glass I is a fluorophosphate glass, and F⁻ and O²⁻ are essentialanion components. For materializing the predetermined optical propertiesand excellent glass stability, it is preferred to introduce 20 to 95% ofF⁻ and 5 to 80% of O²⁻.

Further, when Cl⁻, Br⁻ and I⁻ are introduced in a small amount, thefluorophosphate glass is not easily wetted on platinum products such asa platinum vessel, a nozzle made of platinum, etc., which are used whenthe glass is produced or when the glass is caused to flow out.Therefore, the glass can be easily produced. When Cl⁻, Br⁻ and I⁻ areintroduced to excess, they cause the variation of the refractive indexbecause of the volatilization of a component and the occurrence ofplatinum foreign matter. Therefore, it is preferred to limit the totalcontent of them to 0 to 3%, and it is more preferred to limit the abovetotal content to 0.1 to 3%.

For achieving the object of the present invention, it is preferred toadjust the total content of F⁻, O²⁻, Cl⁻, Br⁻ and I⁻ to 98 anionic % ormore, it is more preferred to adjust the above total content to 99anionic % or more, and it is still more preferred to adjust the abovetotal content to 100 anionic %.

(Optical Glass II)

The optical glass II of the present invention will be explained below.

The optical glass II of the present invention is an optical glass I thatis formed of a fluorophosphate glass having an O²⁻ content/P⁵⁺ contentmolar ratio, O²⁻/P⁵⁺, of 3.5 or more and that has an Abbe's number(ν_(d)) of over 78.

In the optical glass II, the molar ratio of the content of O²⁻ to thecontent of P⁵⁺, O²⁻/P⁵⁺, is preferably 3.55 or more, more preferably 3.6or more.

The optical glass II of the present invention can be said to be oneembodiment of the above optical glass I of the present invention. Whenthe present inventor has made further studies with regard to the aboveoptical glass I of the present invention, the following has been found.In an optical glass formed of a fluorophosphate glass in particularhaving an Abbe's number (ν_(d)) of over 78, a large amount ofmetaphosphoric acid is used as a raw material for introducing anincreased amount of fluorine (F⁻), and it promotes the abovevolatilization of phosphoryl fluoride. The invention of the opticalglass II has been accordingly completed on the basis of the abovefinding.

The optical glass II preferably includes the following optical glassII-a.

The optical glass II-a is a fluorophosphate glass that comprises, ascation components and by cationic %,

3 to 30% of P⁵⁺,

10 to 40% of Al³⁺,

0 to 10% of Mg²⁺,

0 to 30% of Ca²⁺,

0 to 30% of Sr²⁺,

0 to 30% of Ba²⁺,

provided that the total content of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ is 10% ormore,

0 to 30% of Li⁺,

0 to 20% of Na⁺,

0 to 20% of K⁺,

0 to 10% of Y³⁺,

0 to 10% of La³⁺,

0 to 10% of Gd³⁺,

0 to 10% of Yb³⁺,

0 to 10% of B³⁺,

0 to 20% of Zn²⁺ and

0 to 20% of In³⁺,

and comprises, by anionic %,

40 to 95% of F⁻ and

5 to 60% of O²⁻.

P⁵⁺ is an essential component that works as a network former in theglass. When the content thereof is less than 3%, the glass is extremelyunstable. When the content thereof exceeds 30%, it is required foradjusting the O²⁻/P⁵⁺ molar ratio to 3.5 or more to limit the content offluorine to be introduced, and the necessary low-dispersion property canbe no longer obtained. The content of P⁵⁺ is therefore preferablyadjusted to the range of 3 to 30%.

Al³⁺ is an essential component for improving the stability of thefluorophosphate glass. When the content thereof is less than 10%, theglass is destabilized. When the content thereof exceeds 40%, the totalcontent of the other components is too small, and the glass isdestabilized. The content of Al³⁺ is therefore preferably adjusted tothe range of 10 to 40%.

Alkaline earth metals such as Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ are componentsthat improve the stability of the glass and increase the refractiveindex. When the total content thereof is adjusted to 10% or more, theeffect thereof on the stability is increased. However, when the contentof a particular alkaline earth metal component is too large, the balancewith other components is broken. Therefore, it is preferred to introducethem evenly, and it is preferred to introduce at least two members ofMg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺. Specifically, preferably, the content of Mg²⁺is limited to 0 to 10%, the content of Ca²⁺ is limited to 0 to 30%, thecontent of Sr²⁺ is limited to 0 to 30% and the content of Ba²⁺ islimited to 0 to 30%.

Alkali metals such as Li⁺, Na⁺ and K⁺ are components that decrease theglass viscosity and the glass transition temperature to expedite theproduction of the glass. When they are introduced to excess, however,they degrade the glass stability. Therefore, preferably, the content ofLi⁺ is limited to 0 to 30%, the content of Na⁺ is limited to 0 to 20%,and the content of K⁺ is limited to 0 to 20%. Of the alkali metals, Li⁺also has a high effect on improvement of the stability. Therefore, it ismore preferred to introduce 0.5% or more of Li⁺, it is still morepreferred to introduce 1% or more of Li⁺, and it is particularlypreferred to introduce 2% or more of Li⁺.

Rare earth elements such as Y³⁺, La³⁺, Gd³⁺, Yb³⁺, etc., are componentsthat increase the refractive index while maintaining the low-dispersionproperty of the glass. When they are introduced to excess, however, theyincrease the melting temperature of the glass and also decrease theglass stability. It is therefore preferred to limit the content of eachof the above components to 0 to 10%.

B³⁺ is a component that improves the durability of the glass. Since,however, it tends to volatilize as a fluoride during melting, it is alsoa component that decreases productivity. Therefore, it is preferred tolimit the content of B³⁺ to 0 to 10%, it is more preferred to limit thecontent thereof to 0 to 5%, and it is still more preferred to limit thecontent thereof to 0 to 1%. It is particularly preferred to introduce noB³⁺.

Zn²⁺ and In³⁺ have the property that they can be easily introduced tothe glass like alkaline earth metals, and when Zn²⁺ and In³⁺ areintroduced to form a multi-component glass, it is expected that there isproduced an effect that the stability is improved. However, it isundesirable to introduce them to excess. Therefore, it is preferred tolimit the content of each of Zn²⁺ and In³⁺ to 0 to 20%, it is morepreferred to limit the content of each of Zn²⁺ and In³⁺ to 0 to 10%, andit is still more preferred to limit the content of each of Zn²⁺ and In³⁺to 0 to 5%. It is particularly preferred to introduce none of Zn²⁺ andIn³⁺.

In addition to the low-dispersion property and anomalous partialdispersion property, the optical glass II also has the property ofexhibiting a high light transmittance in a broad region fromshort-wavelengths to long-wavelengths in the visible light region. Whenthese properties are to be utilized, the optical glass II is suitable asa material for obtaining various optical elements such as a lens, aprism and the like. For the above use fields, it is preferred not to addions having absorption in the visible light region, e.g., ions of metalelements such as Fe, Cu, Ni, Co, Cr, Mn, V, Nd, Ho and Er.

On the other hand, the optical glass II can be imparted with theproperty of near infrared absorption by adding Cu²⁺. It is hencedesirable to add 0.5 to 13% of Cu²⁺ per 100% of the glass compositionexcluding Cu²⁺. The glass containing Cu²⁺ is suitable as a material forcolor collection filters for semiconductor image sensing devices such asCCD, CMOS, etc. The amount of Cu²⁺ can be property determined in theabove range while taking account of the thickness of the above filters.In the glass containing Cu²⁺, it is desirable not to add any other ionhaving absorption in the visible light region than Cu²⁺ unless theabsorption characteristic is to be adjusted.

Anion components and anion additives will be explained below. Theoptical glass II is a fluorophosphate glass, and F⁻ and O²⁻ areessential anion components. For materializing the predetermined opticalproperties and excellent glass stability, it is preferred to introduce40 to 95% of F⁻ and 5 to 60% of O²⁻.

Further, when Cl⁻, Br⁻ and I⁻ are introduced in a small amount, thefluorophosphate glass is not easily wetted on platinum products such asa platinum vessel, a nozzle made of platinum, etc., which are used whenthe glass is produced or when the glass is caused to flow out.Therefore, the glass can be easily produced. When Cl⁻, Br⁻ and I⁻ areintroduced to excess, they cause the variation of the refractive indexbecause of the volatilization of a component and the occurrence ofplatinum foreign matter. Therefore, it is preferred to limit the totalcontent of them to 0 to 3%, and it is more preferred to limit the abovetotal content to 0.1 to 3%.

For achieving the object of the present invention, it is preferred toadjust the total content of F⁻, O²⁻, Cl⁻, Br⁻ and I⁻ to 98 anionic % ormore, it is more preferred to adjust the above total content to 99anionic % or more, and it is still more preferred to adjust the abovetotal content to 100 anionic %.

The F⁻ content in each of the optical glass I and the optical glass IIis adjusted to 65 anionic % or more for obtaining the property of lowerdispersion. A glass having such a large F⁻ content has very lowviscosity in a molten state and conventionally, it has a problem thatthe occurrence of striae and the variation of the refractive index,which are caused by volatilization, are severe in particular. Accordingto the optical glass I and the optical glass II each of which has an F⁻content of 65 anionic % or more, their property of volatilization issuppressed to a great extent, and the above problem can be henceovercome. Further, not only the property of ultra-low dispersion butalso the property of anomalous dispersion thereof can be increased.

(Optical Glass III)

The optical glass III can be said to be one embodiment of the aboveoptical glass I of the present invention. The optical glass III isformed of a fluorophosphate glass having an O²⁻ content/P⁵⁺ contentmolar ratio, O²⁻/P⁵⁺, of 3.5 or more, having a rare earth elementcontent of less than 5 cationic % and having an F⁻ content/F⁻ and O²⁻total content molar ratio, F⁻/(F⁻+O²⁻), of over 0.2, and has arefractive index (n_(d)) of over 1.53 and an Abbe's number (ν_(d)) ofover 70.

That is, the optical glass III is an optical glass I formed of afluorophosphate glass having a refractive index (n_(d)) of over 1.53,having a rare earth element total content of less than 5 cationic % andhaving an F⁻ content/F⁻ and O²⁻ total content molar ratio, F⁻/(F⁻+O²⁻),of over 0.2. In the optical glass III, the total content of rare earthelements (cationic components of rare earth elements) is preferably 4%or less, more preferably 3% or less. When the total content of the rareearth elements is 5% or more, the melting temperature and the liquidustemperature (molding temperature) of the glass are increased, and theglass is hard to separate or mold as will be described later.

In the optical glass III, the F⁻ content/F⁻ and O²⁻ total content molarratio, F⁻/(F⁻+O²⁻), is preferably 0.3 or more, more preferably 0.4 ormore. When the F⁻ content/F⁻ and O²⁻ total content molar ratio,F⁻/(F⁻+O²⁻), is 0.2 or less, the intended anomalous dispersion propertycan be no longer obtained.

In the optical glass III, the refractive index (n_(d)) is preferably1.54 or more, more preferably 1.55 or more.

The optical glass III of the present invention can be also said to beone embodiment of the above optical glass I of the present invention.When the present inventor made further studies with regard to the aboveoptical glass I of the present invention, the following has been foundand the invention of the optical glass III has been accordinglycompleted on the basis of the finding.

As a high-refractivity low-dispersion fluorophosphate glass having arefractive index (n_(d)) of over 1.53 and an Abbe's number (ν_(d)) ofover 70, there is known a glass having a rare earth metal content of 5cationic % or more. This glass contains a large amount of a rare earthmetal and eventually has both a high melting temperature and a highliquidus temperature (molding temperature). The volatilization amount ofthe above glass component increases with an increase in the temperaturefor causing a molten glass to flow out or the temperature for molding,so that it is preferred to decrease the temperature for causing a moltenglass to flow out or the temperature for molding to the lowest possiblelevel. The above glass containing a large amount of a rare earth elementhas a high melting temperature and a high liquidus temperature (moldingtemperature), and therefore, when an attempt is made to decrease thetemperature for causing a molten glass to flow out or the temperaturefor molding, the viscosity of the glass is high when it is caused toflow out or molded, and it is difficult to separate a glass or mold theglass well. In the optical glass III, therefore, the O²⁻ content/P⁵⁺content molar ratio, O²⁻/P⁵⁺, is limited to 3.5 or more and the totalcontent of rare earth elements is limited to less than 5 cationic %, forsuppressing the volatilization of the glass component.

The optical glass III includes the following optical glass III-a.

The optical glass III-a is a fluorophosphate glass comprising, bycationic %,

20 to 50% of P⁵⁺,

5 to 40% of Al³⁺,

0 to 10% of Mg²⁺,

0 to 20% of Ca²⁺,

0 to 20% of Sr²⁺,

0 to 40% of Ba²⁺,

provided that the total content of Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ is 10% ormore,

0 to 30% of Li⁺,

0 to 20% of Na⁺,

0 to 20% of K⁺,

0 to 5% of Y³⁺, exclusive of 5%,

0 to 5% of La³⁺, exclusive of 5%,

0 to 5% of Gd³⁺, exclusive of 5%,

0 to 5% of Yb³⁺, exclusive of 5%,

provided that the total content of Y³⁺, La³⁺, Gd³⁺ and Yb³⁺ is less than5%,

0 to 10% of B³⁺,

0 to 20% of Zn²⁺ and

0 to 20% of In³⁺.

P⁵⁺ is an essential component that works as a network former in theglass, and it is particularly important for a glass having a relativelysmall fluorine content. When the content of P⁵⁺ is less than 20%, theglass is extremely unstable. When the content thereof exceeds 50%, it isrequired for adjusting the O²⁻/P⁵⁺ molar ratio to 3.5 or more to limitthe content of fluorine to be introduced, and the necessarylow-dispersion property can be no longer obtained. The content of P⁵⁺ istherefore preferably adjusted to the range of 20 to 50%.

Al³⁺ is an essential component for improving the stability of thefluorophosphate glass, and it also has a high effect that a glass havinga small content of fluorine is improved in durability. When the contentof Al³⁺ is less than 5%, the glass is destabilized and greatly degradedin durability. When the content thereof exceeds 40%, the total contentof the other components is too small, and the glass is destabilized. Thecontent of Al³⁺ is therefore preferably adjusted to the range of 5 to40%.

Alkaline earth metals such as Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ are componentsthat improve the stability of the glass and increase the refractiveindex. When the total content thereof is adjusted to 10% or more, theeffect thereof on the stability is increased. However, when the contentof a particular alkaline earth metal component is too large, the balancewith other components is broken. Therefore, it is preferred to introducethem evenly, and it is preferred to introduce at least two members ofMg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺. Further, when much Ba²⁺ is introduced forincreasing the refractive index, the glass having a small fluorinecontent is improved in stability. Specifically, preferably, the contentof Mg²⁺ is limited to 0 to 10%, the content of Ca²⁺ is limited to 0 to20%, the content of Sr²⁺ is limited to 0 to 20% and the content of Ba²⁺is limited to 0 to 40%.

Alkali metals such as Li⁺, Na⁺ and K⁺ are components that decrease theglass viscosity and the glass transition temperature to expedite theproduction of the glass. When they are introduced to excess, however,they degrade the glass stability. Therefore, preferably, the content ofLi⁺ is limited to 0 to 30%, the content of Na⁺ is limited to 0 to 20%,and the content of K⁺ is limited to 0 to 20%. Of the alkali metals, Li⁺also has a high effect on improvement of the stability. Therefore, it ismore preferred to introduce 0.5% or more of Li⁺, it is still morepreferred to introduce 1% or more of Li⁺, and it is particularlypreferred to introduce 2% or more of Li⁺.

Rare earth elements such as Y³⁺, La³⁺, Gd³⁺, Yb³⁺, etc., are componentsthat increase the refractive index while maintaining the low-dispersionproperty of the glass. In a glass having a small fluorine content,however, they are also components that greatly increase the meltingtemperature and the liquidus temperature. It is therefore preferred tolimit the content of each of the above components to 0 to 5% exclusiveof 5%. Further, it is preferred to limit the total content of the aboverare earth elements to less than 5%, it is more preferred to limit theabove total content to 4% or less, and it is still more preferred tolimit the above total content to 3% or less.

B³⁺ is a component that improves the durability of the glass. Since,however, it tends to volatilize as a fluoride during melting, it is alsoa component that decreases productivity. Therefore, it is preferred tolimit the content of B³⁺ to 0 to 10%, and it is more preferred to limitthe content thereof to 0 to 5%. It is still more preferred to introduceno B³⁺.

Zn²⁺ and In³⁺ have the property that they can be easily introduced tothe glass like alkaline earth metals, and when Zn²⁺ and In³⁺ areintroduced to form a multi-component glass, it is expected that there isproduced an effect that the stability is improved. However, it isundesirable to introduce them to excess. Therefore, it is preferred tolimit the content of each of Zn²⁺ and In³⁺ to 0 to 20%, it is morepreferred to limit the content of each of Zn²⁺ and In³⁺ to 0 to 10%, andit is still more preferred to limit the content of each of Zn²⁺ and In³⁺to 0 to 5%. It is particularly preferred to introduce none of Zn²⁺ andIn³⁺.

In addition to the low-dispersion property and anomalous partialdispersion property, the optical glass III also has the property ofexhibiting a high light transmittance in a broad region fromshort-wavelengths to long-wavelengths in the visible light region. Whenthese properties are to be utilized, the optical glass III is suitableas a material for obtaining various optical elements such as a lens, aprism and the like. For the above use fields, it is preferred not to addions having absorption in the visible light region, e.g., ions of metalelements such as Fe, Cu, Ni, Co, Cr, Mn, V, Nd, Ho and Er.

On the other hand, the optical glass III can be imparted with theproperty of near infrared absorption by adding Cu²⁺. It is hencedesirable to add 0.5 to 13% of Cu²⁺ per 100% of the glass compositionexcluding Cu²⁺. The glass containing Cu²⁺ is suitable as a material forcolor collection filters for semiconductor image sensing devices such asCCD, CMOS, etc. The amount of Cu²⁺ can be property determined in theabove range while taking account of the thickness of the above filters.In the glass containing Cu²⁺, it is desirable not to add any other ionhaving absorption in the visible light region than Cu²⁺ unless theabsorption characteristic is to be adjusted.

Anion components and anion additives will be explained below. Theoptical glass III is a fluorophosphate glass, and F⁻ and O²⁻ areessential anion components. Concerning the F⁻ and O²⁻ amount ratio,preferably, the F⁻/(F⁻+O²⁻) is preferably over 0.2 as already described.

Further, when Cl⁻, Br⁻ and I⁻ are introduced in a small amount, thefluorophosphate glass is not easily wetted on platinum products such asa platinum vessel, a nozzle made of platinum, etc., which are used whenthe glass is produced or when the glass is caused to flow out.Therefore, the glass can be easily produced. When Cl⁻, Br⁻ and I⁻ areintroduced to excess, they cause the variation of the refractive indexbecause of the volatilization of a component and the occurrence ofplatinum foreign matter. Therefore, it is preferred to limit the totalcontent of them to 0 to 3%, and it is more preferred to limit the abovetotal content to 0.1 to 3%.

For achieving the object of the present invention, it is preferred toadjust the total content of F⁻, O²⁻, Cl⁻, Br⁻ and I⁻ to 98 anionic % ormore, it is more preferred to adjust the above total content to 99anionic % or more, and it is still more preferred to adjust the abovetotal content to 100 anionic %.

(Optical Glass IV)

The optical glass IV of the present invention will be explained below.

The optical glass IV is an optical glass formed of a fluorophosphateglass comprising P⁵⁺ as a cationic component and F⁻ and O²⁻ as anioniccomponents, the fluorophosphate glass having an F⁻ content of 65 anionic% or more and an O²⁻ content/P⁵⁺ content molar ratio, O²⁻/F⁻, of 3.5 ormore.

In the optical glass IV, the content of F⁻ is adjusted to 65 anionic %or more for materializing the property of ultra-low dispersion. When thecontent of F⁻ is less than 65 anionic %, it is difficult to obtain thedesired low-dispersion property and anomalous dispersion property. Whenthe content of F⁻ is adjusted to 65 anionic % or more, the glass can bealso imparted with a full anomalous dispersion property. The content ofF⁻ is preferably in the range of 65 to 95 anionic %, more preferably inthe range of 80 to 95 anionic %.

Among fluorophosphate glasses, a fluorophosphate glass having a large F⁻content like the optical glass IV has a very low viscosity in its glassmelt state, and in particular, the occurrence of striae and thevariation of a refractive index because of the volatilization areintense.

In the optical glass IV, the generation of a volatile substance per seis suppressed by limiting the O²⁻/P⁵⁺ molar ratio to 3.5 or more, sothat the volatility of the glass is remarkably decreased. Further, thereactivity and corrosiveness of the glass are also suppressed. There canbe therefore produced a high-quality optical glass.

The optical glass IV will be explained with regard to its preferredcompositional ranges. The optical glass IV is preferably afluorophosphate glass comprising, by cationic %,

3 to 15% of P⁵⁺,

25 to 40% of Al²⁺,

5 to 35% of Ca²⁺, and

5 to 25% of Sr²⁺.

The above glass may further comprise, by cationic %,

0 to 10% of Mg²⁺,

0 to 20% of Ba²⁺,

0 to 20% of Li⁺,

0 to 10% of Na⁺,

0 to 10% of K⁺, and

0 to 5% of Y³⁺.

In descriptions hereinafter, “%” for a content of a cationic componentor a total content of cationic components stands for “cationic %”, and“%” for a content of an anionic component or a total content of anioniccomponents stands for “anionic %”, unless otherwise specified.

In the above glass, P⁵⁺ works as a network former. When the content ofP⁵⁺ is less than 3%, the glass stability is degraded. When the contentthereof exceeds 15%, it is required for adjusting the O²⁻/P⁵⁺ molarratio to 3.5 or more to increase the content of O²⁻. As a result, thecontent of F⁻ is decreased, and it is difficult to obtain the fulllow-dispersion property and anomalous dispersion property. The contentof P⁵⁺ is therefore preferably limited to 3 to 15%. The content of P⁵⁺is more preferably in the range of 3.5 to 13%, still more preferably inthe range of 4 to 11%.

Al³⁺ is a component that works to increase the glass stability. When thecontent of Al³⁺ is less than 25%, the glass is destabilized. When thecontent thereof exceeds 40%, the glass is destabilized. The content ofAl³⁺ is therefore preferably limited to 25 to 40%. The content of Al³⁺is preferably in the range of 28 to 33%, more preferably in the range of30 to 36%.

Ca²⁺ has an effect on improvement of the glass stability, and it is acomponent of which the content is desirably increased with an increasein the content of F⁻. When the content of Ca²⁺ is less than 5%, it isdifficult to produce the above effect sufficiently. When it exceeds 35%,the stability is degraded. The content of Ca²⁺ is therefore preferablylimited to 5 to 35%. The content of Ca²⁺ is more preferably in the rangeof 10 to 35%, still more preferably in the range of 20 to 30%.

Sr²⁺ has an effect on improvement of the glass stability. When thecontent thereof is less than 5%, the above effect is insufficient, andwhen it exceeds 25%, the stability is degraded. The content of Sr²⁺ istherefore preferably limited to 5 to 25%. The content of Sr²⁺ is morepreferably in the range of 10 to 25%, still more preferably in the rangeof 15 to 20%.

When Ca²⁺ and Sr²⁺ are allowed to be co-present as described above, theglass is more improved in stability.

When Mg²⁺ is introduced up to 10%, it works to improve the glassstability. It is hence preferred to limit the content of Mg²⁺ to 0 to10%, it is more preferred to limit the above content to 1 to 10%, and itis still more preferred to limit the above content to 3 to 8%.

When Ba²⁺ is introduced up to 20%, it works to improve the glassstability. It is hence preferred to limit the content of Ba²⁺ to 0 to20%. In a glass having a small content of F⁻, Ba²⁺ well works to improvethe stability. However, it is not an essential component for a glasshaving a large content of F⁻. The content of Ba²⁺ is preferably in therange of 1 to 15%, more preferably in the range of 2 to 10%.

For further improving the glass in stability, it is preferred to allowCa²⁺, Sr²⁺ and Mg²⁺ to be co-present, to allow Ca²⁺, Sr²⁺ and Ba²⁺ to beco-present or to allow Ca²⁺, Sr²⁺, Mg²⁺ and Ba²⁺ to be co-present.

Li⁺ decreases the viscosity of a glass melt. However, it works verystrongly to decrease the liquidus temperature. As a whole, it is acomponent that has an effect on the prevention of striae when a moltenglass is caused to flow out and shaped. This effect greatly contributesto an improvement in the quality of the fluorophosphate glass with itssynergistic effect with the effect of the volatile component generationsuppression produced by bringing the O²⁻/P⁵⁺ molar ratio into thepredetermined range. When Li⁺ is introduced in an amount of over 20%, itcauses an excess decrease in the viscosity of a glass melt and causesproblems such as the devitrification of the glass induced by thepromotion of devitrification and the occurrence of striae. The contentof Li⁺ is therefore preferably limited to 0 to 20%. The content of Li⁺is preferably in the range of 0 to 15%, more preferably in the range of1 to 10%, still more preferably in the range of 1 to 7%.

Na⁺ works to decrease the glass transition temperature. However, when itis introduced to excess, the glass is degraded in stability, and it isalso degraded in water resistance. The content of Na⁺ is thereforepreferably limited to 0 to 10%. The content of Na⁺ is preferably in therange of 0 to 7%, more preferably in the range of 1 to 5%.

K⁺ works to decrease the glass transition temperature as well. However,when it is introduced to excess, the glass is degraded in stability, andit is also degraded in water resistance. The content of K⁺ is thereforepreferably limited to 0 to 10%. The content of K⁺ is preferably in therange of 0 to 5%, more preferably in the range of 1 to 3%.

When a plurality of members of Li⁺, Na⁺ and K⁺ as alkali metalcomponents are allowed to be co-present, the glass can be improved instability.

When Y³⁺ is introduced in a small amount, it is expected that the glassstability is improved. When the content thereof exceeds 5%, the meltingtemperature of the glass is increased, and the volatilization from amolten glass is promoted. The glass is also degraded in stability. Thecontent of Y³⁺ is therefore preferably limited to 0 to 5%. The contentof Y³⁺ is preferably in the range of 0 to 5%, more preferably in therange of 1 to 3%.

In addition thereto, La³⁺, Gd³⁺, Zr⁴⁺ and Zn²⁺ may be introduced in asmall amount for adjusting the refractive index.

For obtaining a fluorophosphate glass having excellent molten glassmoldability and a high quality, it is preferred to adjust the totalcontent of P⁵⁺, Al³⁺, Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Na⁺, K⁺ and Y⁺ to 95%or more, it is more preferred to adjust the above total content to 97%or more, it is still more preferred to adjust the above total content to98% or more, and it is yet more preferred to adjust the above totalcontent to 99% or more.

The glass transition temperature of the optical glass IV is preferablylower than 500° C., more preferably 480° C. or lower, still morepreferably 460° C. or lower, yet more preferably 440° C. or lower. Sincehaving the above low glass transition temperature, the optical glass IVis suitable for being precision press-molded, and further, it isexcellent in moldability when it is re-heated and softened. Since theoptical glass IV has the above low glass transition temperature, thetemperature for heating it for molding can be maintained at a relativelylow temperature. Therefore, a reaction between the glass and a mold suchas a press mold does not easily take place, so that glass shapedmaterials having clean and smooth surfaces can be formed. Further, thedeterioration of the mold can be also suppressed.

The Abbe's number (ν_(d)) of the optical glass IV is preferably 85 ormore, more preferably in the range of 88 to 100, still more preferably90 to 97.

The refractive index (n_(d)) thereof is preferably in the range of 1.428to 1.5, more preferably in the range of 1.43 to 1.48.

The optical glass IV has the property of ultra-low dispersion and at thesame time has excellent glass stability represented by a liquidustemperature of 700° C. or lower. There can be therefore provided ahigh-quality fluorophosphate glass as an optical element materialsuitable for color correction.

In the optical glasses I to IV, it is desirable for decreasingenvironmental burdens to introduce none of Pb, As, Cd, Th, etc.Similarly, it is desirable for decreasing environmental burdens tointroduce none of Tl, Te, Cr, Se and U.

The optical glass of the present invention does not require componentslike Lu, Sc, Hf and Ge. Since Lu, Sc, Hf and Ge are expensive, it ispreferred to introduce none of these components.

The optical glass of the present invention exhibits excellent lighttransmittance in a broad wavelength region of the visible light region.When the optical glass of the present invention is not required to haveabsorption in a specific wavelength region while utilizing the aboveproperty, it is preferred to introduce none of substances that causecoloring such as Cu, Cr, V, Fe, Ni, Co, Nd, and the like.

[Process for the Production of Optical Glass]

The process for the production of an optical glass, provided by thepresent invention, will be explained below.

The process for the production of an optical glass, provided by thepresent invention, includes the following three embodiments, i.e., glassproduction processes I to III.

The first embodiment (to be referred to as “glass production process I”hereinafter) of the process for the production of an optical glassformed of a fluorophosphate glass, provided by the present invention,comprises using, as raw materials or cullet, a glass composition havingan a total O²⁻ content/total P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.5 ormore when the optical glass is produced by melting the raw materials orcullet and refining and homogenizing a molten glass, and therebyproducing the optical glass of the present invention.

That is, the glass production process I is a process for the productionof the optical glass of the present invention, in which a glasscomposition having an a total O²⁻ content/total P⁵⁺ content molar ratio,O²⁻/P⁵⁺, of 3.5 or more is used as raw materials or cullet when theoptical glass is obtained by melting the raw materials or cullet andthen refining and homogenizing the molten glass.

The second embodiment (to be referred to as “glass production processII” hereinafter) of the process for the production of an optical glass,provided by the present invention, is a process for the production of anoptical glass formed of a fluorophosphate glass, which comprisespreparing a raw material batch from raw materials or cullet, melting theraw material batch and then carrying out refining and homogenization,

the process comprising preparing the raw material batch in which thetotal O²⁻ content/total P⁵⁺ content molar ratio, O²⁻/P⁵⁺, is 3.5 or moreand carrying out the melting, refining and homogenization to produce afluorophosphate glass having an Abbe's number (ν_(d)) of over 70.

In a glass having an O²⁻ content/P⁵⁺ content molar ratio, O²⁻/P⁵⁺, ofless than 3.5, a volatile substance is generated during the melting ofthe glass and a glass component volatilizes during the production of theglass as already described. Therefore, in the glass production processI, a glass composition having an a total O²⁻ content/total P⁵⁺ contentmolar ratio, O²⁻/P⁵⁺, of 3.5 or more is used as raw materials or cullet,and in the glass production process II, there is prepared a raw materialbatch in which the total O²⁻ content/total P⁵⁺ content molar ratio,O²⁻/P⁵⁺, is 3.5 or more, whereby the generation of a volatile substanceis suppressed during the melting of each glass and the volatilization ofa component is suppressed during the production of each glass.

For bringing the total O²⁻ content/total P⁵⁺ content molar ratio,O²⁻/P⁵⁺, in the glass raw materials or cullet into 3.5 or more, it ispreferred to use, as one of glass raw materials, a diphosphate in whichthe ratio of number of oxygen (O²⁻) atoms per atom of phosphorus (P⁵⁺)(oxygen atom/phosphorus atom) is 3.5 or to use a cullet prepared fromglass raw materials including the diphosphate.

In the glass production processes I and II, for example, glass rawmaterials such as phosphate, fluoride, etc., which are properly weighedand mixed so as to give a desired composition having a total O²⁻content/total P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.5 or more, are fedto a melting vessel made of platinum alloy, heated and melted, and amolten glass is refined, homogenized and then caused to flow out of apipe, followed by shaping, whereby an optical glass having desiredproperties can be obtained.

The third embodiment (to be referred to as “glass production processIII” hereinafter) of the process for the production of an optical glass,provided by the present invention, is a process for the production of anoptical glass formed of a fluorophosphate glass, which comprisespreparing a raw material batch from raw materials or cullet, melting theraw material batch, then carrying out refining and homogenization toprepare a molten glass and shaping said molten glass,

the process comprising controlling the total O²⁻ content/total P⁵⁺content molar ratio, O²⁻/P⁵⁺, in said raw material batch for decreasingthe volatility of said molten glass.

In the glass production process III, on the basis of a novel findingthat the O²⁻/P⁵⁺ molar ratio has a great effect on the volatility of amolten glass, the total O²⁻ content/total P⁵⁺ content molar ratio,O²⁻/P⁵⁺, is controlled so that the volatility of a molten glass isdecreased, to produce an optical glass.

That is, the above molar ratio is so controlled that the refractiveindex (n_(d)) and Abbe's number (ν_(d)) come to be desired values andthat the volatility of a molten glass is decreased. The above molarratio is adjusted such that it is 3.5 or more. The above molar ratio ispreferably in the above-described range.

In the glass production processes I to III, the procedures of heatingand melting the glass raw materials or cullet are preferably carried outin the atmosphere of an inert gas such as nitrogen gas or the like. As amelting apparatus for melting the glass, a known melting apparatus forfluorophosphate glass can be used.

Like the glass production process I, the glass production processes IIand III are suitable for producing a fluorophosphate glass having anAbbe's number (ν_(d)) of over 70, a fluorophosphate glass having anAbbe's number (ν_(d)) of over 78, a fluorophosphate glass in which thetotal content of rare earth elements is less than 5 cationic % and theF⁻ content/F⁻ and O²⁻ total content molar ratio, F⁻/(F⁻+O²⁻), is over0.2 and which has a refractive index (n_(d)) of over 1.53 and afluorophosphate glass having an F⁻ content of 65 anionic % or more.

[Press-Molding Preform and Process for the Production Thereof]

The press-molding preform of the present invention will be explainedbelow.

The press-molding preform of the present invention is characteristicallyformed of the optical glass of the present invention or an optical glassobtained by the process for the production of an optical glass, providedby the present invention.

The press-molding preform as used herein means a material obtained bypre-shaping a glass having a weight equivalent to the weight of apress-molded article to a form suitable for press-molding.

The press-molding preform of the present invention is in particularsuitable for precision press-molding, and when it is used a precisionpress-molding preform, it is preferred to form a mold release film suchas a carbon film, or the like on the entire surface of the preform.

The process for the production of a press-molding preform, provided bythe present invention, will be explained below.

The process for the production of a press-molding preform, provided bythe present invention, includes two embodiments.

The first embodiment (to be referred to as “preform production processI” hereinafter) of the process for the production of a preform, providedby the present invention, is a process for the production of apress-molding preform which comprises causing a molten glass to flow outof a pipe to separate a molten glass gob having a predetermined weightand shaping said glass gob into a preform when the glass goes through aprocess of cooling, the process comprising shaping the press-moldingpreform of the present invention.

That is, the preform production process I is a process for theproduction of the press-molding preform of the present invention, and itis a process comprising causing a molten glass to flow out of a pipe toseparate a molten glass gob having a predetermined weight and shapingthis glass gob into a preform when the glass goes through a process ofcooling.

In the preform production process I, first, a molten glass is caused toflow out of a pipe. For example, a molten glass is caused to flow,continuously at a constant rate, out of a pipe made of a platinum alloyor platinum which pipe is heated to a predetermined temperature by aheating method such as an electric heating method, a high-frequencyinduction heating method or a combination of these methods.

Then, a molten glass gob having the weight of one preform or the weightobtained by adding a removal amount to be described later to the weightof one preform is separated from the molten glass that is flowing out.For separating the molten glass gob, desirably, no cutting blade is usedso as not to leave a cutting mark. For example, it is preferred toemploy a method in which the molten glass is caused to fall as dropsfrom the flow outlet of the pipe or a method in which the forward end ofthe molten glass that is flowing out is supported with a support and thesupport is rapidly moved downward at a time when the molten glass gobhaving the intended weight is separable, to separate the molten glassgob from the forward end portion of the flowing molten glass byutilizing the surface tension of the molten glass.

The thus-separated molten glass gob is shaped on/above the concaveportion of a preform shaping mold into a preform having a desired formwhen the glass goes through a process of cooling. In the case,preferably, the shaping is carried out in a state in which the glass iscaused to float above the concave portion by applying an upward gaspressure to the glass gob, for preventing the formation of wrinkles onthe preform surface of the breaking called cracking during the processof glass cooling.

After the glass is cooled to a temperature at which the preform is notdeformable even by application of an external force, the preform istaken out of the shaping mold and gradually cooled.

The thus-obtained preform is originally formed of the glass that doesnot easily cause striae as described above. However, when striae areslightly formed on the preform surface, the striae are present locallyin a preform surface layer, so that an optically highly uniform preformfree of striae can be completed by removing the above surface layer byan etching or polishing process.

When any one of the etching process and the polishing process isemployed, desirably, the molten glass gob having the weight obtained byadding a removal amount to the intended preform weight is separated suchthat the glass gob comes to have the intended weight after the surfacelayer is removed.

The preform production process I is in particular suitable as a processfor the production of precision press-molding preforms.

The second embodiment (to be referred to as “preform production processII” hereinafter) of the process for the production of a preform,provided by the present invention, is a process for the production of apress-molding preform, which comprises casting a molten glass into acasting mold to prepare a glass shaped material and processing saidglass shaped material to make the press-molding preform,

the process comprising shaping the press-molding preform of the presentinvention.

The above casting mold can be selected from known ones as requireddepending upon a shaping form. For example, a casting mold having a flatbottom surface, three side walls surrounding the bottom surface on threesides of the bottom and one open side is arranged below the pipe thatallows a molten glass to flow out, in a manner that the bottom surfaceis horizontal. And, the molten glass flowing continuously out of thepipe is cast on the bottom surface of the casting mold and shaped in aplate form while the glass is filled in a portion surrounded with theside walls. A shaped glass is withdrawn from the above opening portionat a constant speed in the horizontal direction, to obtain a glass platehaving a constant width and a constant thickness. The thus-withdrawnglass plat is annealed while it slowly passes through an annealingfurnace. The annealed glass plate is cut at right angles with thedrawing direction, to obtain a glass plate having a desired length.

There may be employed a constitution in which a casting mold having athrough hole is arranged below the pipe that allows a molten glass toflow out, in a manner that the through hole extends vertically, and themolten glass is caused to continuously flow into the through hole. Theglass that is caused to flow into the through hole is rapidly cooled andshaped in a rod form, and the shaped glass is withdrawn in the downwarddirection at a constant speed from the lower end opening portion of thethrough hole. The glass rod withdrawn from the casting mold passesthrough an atmosphere that is heated to a temperature around the glasstransition temperature thereof, and after the glass rod is subjected toprocedures that bring the surface temperature and inside temperature ofthe glass rod close to each other, it was cut in the horizontaldirection, to give a glass rod having a desired length.

The thus-obtained glass shaped material in the form of a plate or a rodis divided into glass pieces by cutting or splitting, and each glasspiece is mass-adjusted by barrel polishing so as to have a mass havingthe weight of one optical element blank, whereby a press-molding preformis obtained. By the barrel polishing, edges of the glass piece can berounded and the edges that may cause a breaking or a folding duringpress molding can be removed. Further, the preform is surface-roughenedso that a mold release agent in the form of a powder easily adheres tothe surface during its press molding. Differing from a precisionpress-molded article, the thus-obtained preform is a glass materialwhich is press-molded to produce an optical element blank which isthereafter surface-ground and polished to produce an optical elementhaving an optical function surface.

Another example is a method in which the above glass piece is ground andpolished so that the glass surface is smoothened to form a precisionpress-molding preform, and a still another example is a method in whichthe above barrel-polished product is surface-smoothened by polishing toform a precision press-molding preform.

[Optical Element Blank and Process for the Production Thereof]

The optical element blank of the present invention will be explainedbelow.

The optical element blank of the present invention is characteristicallyformed of the optical glass of the present invention or an optical glassobtained by the process of the present invention.

The optical element blank is a glass shaped product from which anoptical element is completed by cutting and grinding as described above,and it has the form obtained by acing a process margin to be removed bycutting and grinding to the form of an intended optical element, thatis, a form similar to the form of an optical element.

The process for the production of an optical element blank, provided bythe present invention, will be explained below.

The process for the production of an optical element, provided by thepresent invention, includes two embodiments.

The first embodiment (to be referred to as “optical element blankproduction method I” hereinafter) is a process for the production of anoptical element blank from which an optical element is completed bycutting and grinding, the process comprising heating and press-moldingthe preform of the present invention or a preform obtained by theprocess of the present invention.

In this process, the heating is preceded by uniformly applying a moldrelease agent in the form of a powder such as boron nitride to thepreform surface and placing the preform on a refractory plate andintroducing the refractory plate with the preform on it into aheating-softening furnace, and the preform is heated until the glass issoftened. The preform is introduced into a press mold and pressed. Then,the resultant press-molded product is taken out of the mold and annealedto remove a strain and to adjust optical properties in order to ensurethat optical properties such as a refractive index come to be desiredvalues. In this manner, an optical element blank can be produced.

The second embodiment (to be referred to as “optical element blankproduction method II” hereinafter) of the process for the production ofan optical element, provided by the present invention, is a process forthe production of an optical element blank which comprises melting glassraw materials, causing the resultant molten glass to flow out,separating a molten glass gob from a molten glass flow and press-moldingthe molten glass gob, the process comprising melting and molding theoptical glass of the present invention or an optical glass obtained bythe process of the present invention.

In this process, a homogenized molten glass is caused to flow out on themolding surface of a lower mold member to which a mold release agent inthe form of a powder such as boron nitride is applied, and the moltenglass flow having its lower end portion supported on the lower moldmember is cut off with a cutting blade called shears. In this manner,the molten glass gob having a desired mass is obtained on the moldingsurface of the lower mold member. Then, the lower mold member with themolten glass gob on it is transferred right below an upper mold memberstanding in a different position, and the molten glass gob is pressedwith the upper and lower mold members to form an optical element blank.The resultant press-molded product is taken out of the mold and annealedto remove a strain and to adjust optical properties in order to ensurethat optical properties such as a refractive index come to be desiredvalues. In this manner, an optical element blank can be produced.

Both the optical element production processes I and II can be carriedout in atmosphere. The molding conditions, the material of the pressmold, the heating-softening furnace and the refractory plate used forthe heating and softening, etc., can be selected from known conditionsor those which are known.

According to the present invention, there can be provided an opticalelement blank from which an optical element free of defects such asstriae, etc., can be produced, and a process for the production thereof.

[Optical Element and Process for the Production Thereof]

The optical element of the present invention will be explained below.

The optical element of the present invention is characteristicallyformed of the optical glass of the present invention or an optical glassobtained by the process of the present invention.

The optical element of the present invention is formed of the aboveoptical glass of the present invention or an optical glass obtained bythe above process of the present invention, there can be providedoptical elements utilizing the low dispersion property.

While the optical element is not specially limited with regard to itskind and form, examples thereof include an aspherical lens, a sphericallens, a micro-lens, a lens array, a prism, a diffraction grating, aprism with a lens, a lens with a diffraction grating, etc. Examples ofthe aspherical lens and spherical lens include a convex meniscus lens, aconcave meniscus lens, a biconvex lens, biconcave lens, a plano-convexlens, a plano-concave lens, etc.

In view of use fields, examples of the optical element include a lensfor a digital camera, a lens for a cellphone with a built-in camera, anoptical pickup lens, a collimator lens, a lens for opticalcommunications, etc.

The surface of the optical element may be provided with an optical thinfilm such as an anti-reflection film, or the like.

The process for the production of an optical element, provided by thepresent invention, will be explained below.

The process for the production of an optical element, provided by thepresent invention, includes two embodiments.

The first embodiment (to be referred to as “optical element productionprocess I” hereinafter) is a process for the production of an opticalelement, which comprises cutting and grinding the optical element blankof the present invention or an optical element blank produced by theprocess of the present invention.

For the above cutting and grinding, known methods can be employed. Theoptical element production process I is suitable for producing opticalelements easily produced by cutting and grinding and a large-diameterlens such as the most object side lens of telescopic lenses.

The second embodiment (to be referred to as “optical element productionprocess II” hereinafter) of the process for the production of an opticalelement, provided by the present invention, comprises heating andprecision press-molding the preform of the present invention or apreform obtained by the process of the present invention. That is, theoptical element production method II is a process for the production ofthe optical element of the present invention, which comprises heatingand precision press-molding the preform of the present invention or apreform obtained by the process of the present invention.

The above precision press-molding is called “optics molding” as well andis well known in the field of this art. In the optical element, asurface which transmits, refracts, diffracts or reflects light isreferred to as an optical function surface (for example, the asphericalsurface of an aspherical lens, the spherical surface of a sphericallens, etc., correspond to the optical function surface). According toprecision press-molding, the form of the molding surface of a press moldis precisely transferred to a glass, whereby the optical functionsurface can be formed by press-molding, and machine procedures such ascutting and grinding are not required for completing the opticalfunction surface.

The process for the production of an optical element, provided by thepresent invention, is therefore suitable for the production of opticalelements such as a lens, a lens array, a diffraction grating, a prism,etc., and in particular suitable as a process for highly productivelyproducing aspherical lenses.

The press mold for the precision press-molding can be selected fromknown press molds, such as press molds obtained by forming a moldrelease film on the molding surface of a mold material made ofrefractory ceramic such as silicon carbide, zirconia, alumina, or thelike. Of these, a press mold made of silicon carbide is preferred, and acarbon-containing film can be used as a mold release film. In view ofdurability and a cost, a carbon film is particularly preferred.

In the precision press-molding, desirably, a non-oxidizing gasatmosphere is employed as an atmosphere during molding for maintainingthe molding surface of the press mold under excellent conditions. Thenon-oxidizing gas is preferably selected from nitrogen, a mixture gas ofnitrogen and hydrogen, or the like.

The precision press-molding for use in the process for the production ofan optical element, provided by the present invention, includes thefollowing two embodiments of precision press-molding I and precisionpress-molding II.

(Precision Press-Molding I)

The precision press-molding I is a method in which a preform isintroduced into a press mold and the press mold and the preform areheated together to carry out the precision press-molding.

In the precision press-molding I, preferably, the press mold and theabove preform are heated to a temperature at which the glassconstituting the preform exhibits a viscosity of 10⁶ to 10¹² dPa·s tocarry out the precision press-molding.

Further, desirably, a precision press-molded product is cooled to atemperature at which the above glass preferably exhibits a viscosity of10¹² dPa·s or more, more preferably 10¹⁴ dPa·s or more, still morepreferably 10¹⁶ dPa·s or more before it is taken out of the press mold.

Under the above conditions, not only the form of the molding surface ofthe press mold can be precisely transferred to the glass, but also theprecision press-molded product can be taken out without any deformation.

(Precision Press-Molding II)

The precision press-molding II is a method in which a preform heated isintroduced to a pre-heated press mold to carry out the precisionpress-molding.

According to this precision press-molding II, preforms are pre-heatedbefore they are introduced into a press mold, so that optical elementsthat are free of surface defects and that have excellent surfaceaccuracy can be produced in an optical element production cycle that isshortened.

The temperature for pre-heating the press mold is preferably set at atemperature lower than the temperature for pre-heating the preform. Whenthe temperature for pre-heating the press mold is so decreased, theabrasion of the press mold can be decreased.

In the precision press-molding II, the above preform is preferablypre-heated to a temperature at which the glass constituting the preformexhibits a viscosity of 10⁹ dPa·s or less, more preferably, 10⁹ dPa·s.

Further, preferably, the above preform is pre-heated while it is causedto float. Further, the above preform is more preferably pre-heated to atemperature at which the glass constituting the preform exhibits aviscosity of 10^(5.5) to 10⁹ dPa·s, and it is still more preferablypre-heated to a temperature at which the glass constituting the preformexhibits a viscosity of 10^(5.5) or more but less than 10⁹ dPa·s.

Preferably, the cooling of the glass is started upon the start ofpressing or during the pressing.

In addition, the press mold is temperature-adjusted to a temperaturelower than the pre-heating temperature for the preform, while thetemperature adjustment can be made using, as a target, a temperature atwhich the above glass exhibits a viscosity of 10⁹ to 10¹² dPa·s.

In the above process, preferably, a press-molded product is taken out ofthe mold after it is cooled to a temperature at which the above glassexhibits a viscosity of 10¹² dPa·s or more.

The optical element obtained by the precision press-molding is taken outof the press mold and cooled as required. When the molded product is anoptical element such as a lens or the like, an optical thin film may becoated on the surface thereof as required.

EXAMPLES

The present invention will be more specifically explained with referenceto Examples hereinafter, while the present invention shall not belimited by these Examples.

Example 1 and Comparative Example 1 Production Examples of OpticalGlasses

For producing optical glasses Nos. 1 to 38 having compositions shown inTables 1-1 to 1-8 and optical glasses Nos. 1 and 2 having compositionsshown in Table 1-2, phosphates such as diphosphate, etc., and rawmaterials such as fluoride, corresponding to components of each glass,were weighed and fully mixed. Tables 1-1 to 1-8 also show a ratio of atotal content of O²⁻ to a total content of P⁵⁺ (O²⁻/P⁵⁺), a contentratio of rare earth meals (cationic %) and a ratio of the content of F⁻to the total content of F⁻ and O²⁻ (F⁻/(F⁻+O²⁻)) in each of raw materialbatches. The above raw material batch was charged into a platinumcrucible and melted under heat in an electric furnace at 900° C. withstirring over the time period of 1 to 3 hours, and a molten glass wasrefined and homogenized. In this manner, the optical glasses Nos. 1 to38 and comparative optical glasses Nos. 1 and 2 were obtained. In Tables1-1 to 1-8, the optical glasses Nos. 1 to 4 correspond to the opticalglasses I and II of the present invention, the optical glasses Nos. 5 to9 correspond to the optical glasses I and III of the present invention,and the optical glasses Nos. 10 to 38 correspond to the optical glass IVof the present invention.

In the optical glasses Nos. 1 to 38, the ratio of the total content ofO²⁻ to the total content of P⁵⁺ (O²⁻/P⁵⁺) each was controlled so that itwas 3.5 or more as shown in Tables 1-1 to 1-8, and the contents of theother components were balanced, whereby there were obtained opticalglasses whose volatility was greatly decreased and which had desiredproperties. While the above Production Examples used non-vitrified rawmaterials including phosphates such as diphosphate, etc., and fluorides,cullet may be used or non-vitrified raw materials and cullet may be usedin combination.

With regard to each of the above optical glasses and comparative opticalglasses, 200 g of a sample obtained by melting raw materials for 1 hourwere measured for a refractive index (n_(d)) (1 h) and an Abbe's number(ν_(d)) (1 h), and 200 g of a sample obtained by melting raw materialsfor 3 hours was measured for a refractive index (n_(d)) (3 h) and anAbbe's number (ν_(d)) (3 h) and measured for a glass transitiontemperature. Tables 1-1 and 1-8 show the results.

The refractive indices (n_(d)), Abbe's numbers (ν_(d)) and glasstransition temperatures (T_(g)) of the above optical glasses weremeasured by the following methods.

(1) Refractive Index (n_(d)) and Abbe's Number (ν_(d))

An optical glass obtained by adjusting a gradual cooling rate at −30°C./hour was measured.

(2) Glass Transition Temperature (T_(g))

Measurement was made with an apparatus for thermomechanical analysissupplied by Rigaku Corporation (Thermoplus TMA 8310) by setting atemperature elevation rate at 4° C./minute.

TABLE 1-1 Optical glass 1 2 3 4 5 Cationic components (cationic %) P⁵⁺19.0 20.3 20.0 19.7 32.6 Al³⁺ 22.7 22.3 22.4 22.5 11.6 Mg²⁺ 6.8 6.7 6.76.8 6.3 Ca²⁺ 8.5 8.4 8.4 8.5 6.3 Sr²⁺ 14.5 14.3 14.3 14.4 5.3 Ba²⁺ 10.110.0 10.0 10.0 16.9 Me²⁺ + Ca²⁺ + Sr²⁺ + Ba²⁺ 39.9 39.4 39.4 39.7 34.8Li⁺ 17.3 17.0 17.1 17.1 20.0 Na⁺ 0.0 0.0 0.0 0.0 0.0 K⁺ 0.0 0.0 0.0 0.00.0 Y³⁺ 1.1 1.0 1.1 1.0 1.0 La³⁺ 0.0 0.0 0.0 0.0 0.0 Gd³⁺ 0.0 0.0 0.00.0 0.0 Yb³⁺ 0.0 0.0 0.0 0.0 0.0 Y³⁺ + La³⁺ + Gd³⁺ + Yb³⁺ 1.1 1.0 1.11.0 1.0 B³⁺ 0.0 0.0 0.0 0.0 0.0 Zn²⁺ 0.0 0.0 0.0 0.0 0.0 In³⁺ 0.0 0.00.0 0.0 0.0 Cation total 100.0 100.0 100.0 100.0 100.0 Anioniccomponents F⁻ (anionic %) 62.9 62.0 61.8 61.7 35.1 O²⁻ (anionic %) 37.138.0 38.2 38.3 64.9 F⁻/(F⁻ + O²⁻) 0.629 0.620 0.618 0.617 0.351 O²⁻/P⁵⁺3.74 3.61 3.67 3.72 3.50 Refractive index (n_(d)) Nd (1 h) 1.495821.49504 1.49649 1.49817 1.55042 Nd (3 h) 1.49630 1.49605 1.49733 1.498881.55147 Nd (3 h) − Nd (1 h) 0.00048 0.00101 0.00084 0.00071 0.00105Abbe's number (ν_(d)) νd (1 h) 82.0 81.7 81.4 81.3 71.9 νd (3 h) 81.981.6 81.3 81.3 71.9 −0.1 −0.1 −0.1 0.0 0.0 Glass transition 389temperature (° C. (Notes) Nd (1 h) shows a refractive index (n_(d)) of asample obtained by melting at 900° C. for 1 hour. Nd (3 h) shows arefractive index (n_(d)) of a sample obtained by melting at 900° C. for3 hours. ν_(d) (1 h) shows an Abbe's number (ν_(d)) of a sample obtainedby melting at 900° C. for 1 hour. ν_(d) (3 h) shows an Abbe's number(ν_(d)) of a sample obtained by melting at 900° C. for 3 hours.

TABLE 1-2 Comparative Optical glass Optical Glass 6 7 8 9 1 2 Cationiccomponents (cationic %) P⁵⁺ 29.0 31.9 32.6 30.0 25.0 25.0 Al³⁺ 9.0 11.711.6 12.0 21.0 21.0 Mg²⁺ 6.0 6.4 6.3 6.6 6.3 6.3 Ca²⁺ 4.0 6.4 6.3 6.67.9 7.9 Sr²⁺ 5.0 5.3 5.3 5.5 13.4 13.4 Ba²⁺ 25.0 17.0 16.9 17.5 9.4 9.4Mg²⁺ + Ca²⁺ + Sr²⁺ + Ba²⁺ 40.0 35.1 34.8 36.2 37.0 37.0 Li⁺ 21.0 20.220.0 20.8 16.0 16.0 Na⁺ 0.0 0.0 0.0 0.0 0.0 0.0 K⁺ 0.0 0.0 0.0 0.0 0.00.0 Y³⁺ 1 1.1 1.0 1.0 1.0 1.0 La³⁺ 0.0 0.0 0.0 0.0 0.0 0.0 Gd³⁺ 0.0 0.00.0 0.0 0.0 0.0 Yb³⁺ 0.0 0.0 0.0 0.0 0.0 0.0 Y³⁺ + La³⁺ + Gd³⁺ + Yb³⁺1.0 1.1 1.0 1.0 1.0 1.0 B³⁺ 0.0 0.0 0.0 0.0 0.0 0.0 Zn²⁺ 0.0 0.0 0.0 0.00.0 0.0 In³⁺ 0.0 0.0 0.0 0.0 0.0 0.0 Cation total 100.0 100.0 100.0100.0 100.0 100.0 Anionic components F⁻(anionic %) 41.3 34.6 35.1 39.463.4 58.4 O²⁻(anionic %) 58.7 65.4 64.9 60.6 36.6 41.6 F⁻/(F⁻ + O²⁻)0.413 0.346 0.351 0.394 0.634 0.584 O²⁻/P⁵⁺ 3.51 3.56 3.50 3.54 3.003.29 Refractive index (n_(d)) Nd(1 h) 1.54837 1.55226 1.54865 1.544451.48806 1.49574 Nd(3 h) 1.54910 1.55295 1.54979 1.54527 1.49288 1.50097Nd(3 h) − Nd(1 h) 0.00073 0.00069 0.00114 0.00082 0.00482 0.00523 Abbe'snumber (ν_(d)) νd(1 h) 71.8 71.7 72.0 72.6 82.4 80.9 νd(3 h) 71.6 71.771.8 72.9 81.3 80.4 νd(3 h) − νd(1 h) −0.2 0.0 −0.2 0.3 −1.1 −0.5 Glasstransition 369 395 389 385 378 temperature (° C.) (Notes) Nd(1 h) showsa refractive index (n_(d)) of a sample obtained by melting at 900° C.for 1 hour. Nd(3 h) shows a refractive index (n_(d)) of a sampleobtained by melting at 900° C. for 3 hours. ν_(d) (1 h) shows an Abbe'snumber (ν_(d)) of a sample obtained by melting at 900° C. for 1 hour.ν_(d) (3 h) shows an Abbe's number (ν_(d)) of a sample obtained bymelting at 900° C. for 3 hours.

TABLE 1-3 Optical glass No. 10 11 12 13 14 15 Cationic % P⁵⁺ 11.17 11.1711.17 11.67 11.17 11.17 Al³⁺ 27.09 28.09 28.09 31.59 32.08 32.09 Mg²⁺4.07 4.07 4.07 4.07 4.07 4.07 Ca²⁺ 23.26 23.26 23.26 23.26 25.00 23.26Sr²⁺ 15.09 15.09 15.09 15.09 16.09 15.09 Ba²⁺ 8.52 8.52 8.52 8.52 5.798.52 Li⁺ 8.12 7.12 7.12 3.12 3.12 3.12 Na⁺ 0.00 0.00 0.00 0.00 0.00 0.00K⁺ 0.00 0.00 0.00 0.00 0.00 0.00 Y³⁺ 2.68 2.68 2.68 2.68 2.68 2.68 Zn²⁺0.00 0.00 0.00 0.00 0.00 0.00 Anionic % O²⁻ 19.34 19.16 18.47 18.1517.82 17.82 Cl⁻ 0.19 0.19 0.18 0.18 0.18 0.18 F⁻ 80.47 80.65 81.35 81.6782.00 82.00 O²⁻/P⁵⁺ 3.7 3.7 3.59 3.5 3.59 3.59 N_(d) 1.46545 1.464661.46196 1.45886 1.45599 1.45869 ν_(d) 88.3 88.7 89.2 90 90.5 90.1 Tg (°C.) 389 390 393 425 420 424 LT (° C.) 650 650 650 620 610 620 Nd(1 h)1.46545 1.46466 1.46196 1.45886 1.45599 1.45869 Nd(3 h) 1.46570 1.463461.46067 1.45989 Nd(3 h) − Nd(1 h) 0.00104 0.00150 0.00181 0.00120 |Nd(3h) − Nd(1 h)| 0.00104 0.00150 0.00181 0.00120 (Notes) Nd(1 h) shows arefractive index (n_(d)) of a sample obtained by melting at 900° C. for1 hour. Nd(3 h) shows a refractive index (n_(d)) of a sample obtained bymelting at 900° C. for 3 hours. ν_(d) (1 h) shows an Abbe's number(ν_(d)) of a sample obtained by melting at 900° C. for 1 hour. ν_(d) (3h) shows an Abbe's number (ν_(d)) of a sample obtained by melting at900° C. for 3 hours.

TABLE 1-4 Optical glass No. 16 17 18 19 20 Cationic % P⁵⁺ 11.17 11.4410.25 11.17 11.17 Al³⁺ 32.09 31.82 29.08 32.09 34.09 Mg²⁺ 4.07 4.20 4.074.07 4.07 Ca²⁺ 23.26 23.13 23.26 23.26 23.26 Sr²⁺ 15.09 15.09 15.0915.09 15.09 Ba²⁺ 8.52 8.52 9.45 8.52 8.52 Li⁺ 3.12 3.12 6.12 3.12 3.12Na⁺ 0.00 0.00 0.00 0.00 0.00 K⁺ 0.00 0.00 0.00 0.00 0.00 Y³⁺ 2.68 2.682.68 2.68 0.68 Zn²⁺ 0.00 0.00 0.00 0.00 0.00 Anionic % O²⁻ 17.82 17.7617.36 17.32 17.32 Cl⁻ 0.18 0.18 0.18 0.18 0.18 F⁻ 82.00 82.06 82.4682.50 82.50 O²⁻/P⁵⁺ 3.59 3.5 3.7 3.5 3.5 N_(d) 1.45936 1.45832 1.462331.45729 1.45305 ν_(d) 90.6 90.4 89.6 90.4 91.2 Tg (° C.) 424 420 400 415426 LT (° C.) 620 610 650 620 650 Nd(1 h) 1.45936 1.45832 1.462331.45729 1.45305 Nd(3 h) 1.46048 1.45880 Nd(3 h) − Nd(1 h) 0.001120.00151 |Nd(3 h) − Nd(1 h)| 0.00112 0.00151 (Notes) Nd(1 h) shows arefractive index (n_(d)) of a sample obtained by melting at 900° C. for1 hour. Nd(3 h) shows a refractive index (n_(d)) of a sample obtained bymelting at 900° C. for 3 hours. ν_(d) (1 h) shows an Abbe's number(ν_(d)) of a sample obtained by melting at 900° C. for 1 hour. ν_(d) (3h) shows an Abbe's number (ν_(d)) of a sample obtained by melting at900° C. for 3 hours.

TABLE 1-5 Optical glass No. 21 22 23 24 25 Cationic % P⁵⁺ 11.17 6.806.17 6.00 5.42 Al³⁺ 32.09 35.80 36.09 35.80 33.70 Mg²⁺ 4.07 4.30 3.074.30 6.83 Ca²⁺ 23.26 23.70 25.38 24.50 28.72 Sr²⁺ 15.09 18.40 15.0918.40 17.16 Ba²⁺ 8.52 6.00 8.52 6.00 4.70 Li⁺ 3.12 2.30 2.00 2.30 1.00Na⁺ 0.00 0.00 0.00 0.00 1.20 K⁺ 0.00 0.00 0.00 0.00 0.00 Y³⁺ 2.68 2.703.68 2.70 1.27 Zn²⁺ 0.00 0.00 0.00 0.00 0.00 Anionic % O²⁻ 17.32 10.229.21 9.01 8.43 Cl⁻ 0.18 0.17 0.17 0.17 0.00 F⁻ 82.50 89.61 90.62 90.8291.57 O²⁻/P⁵⁺ 3.5 3.5 3.5 3.5 3.57 N_(d) 1.45762 1.43915 1.43821 1.436961.43284 ν_(d) 90.4 94.9 95.5 95.2 93.2 Tg (° C.) 422 410 418 LT (° C.)600 650 650 670 650 Nd(1 h) 1.45762 1.43915 1.43821 1.43696 1.43284 Nd(3h) 1.44045 1.43455 Nd(3 h) − Nd(1 h) 0.00130 0.00171 |Nd(3 h) − Nd(1 h)|0.00130 0.00171 (Notes) Nd(1 h) shows a refractive index (n_(d)) of asample obtained by melting at 900° C. for 1 hour. Nd(3 h) shows arefractive index (n_(d)) of a sample obtained by melting at 900° C. for3 hours. ν_(d) (1 h) shows an Abbe's number (ν_(d)) of a sample obtainedby melting at 900° C. for 1 hour. ν_(d) (3 h) shows an Abbe's number(ν_(d)) of a sample obtained by melting at 900° C. for 3 hours.

TABLE 1-6 Optical glass No. 26 27 28 29 30 Cationic % P⁵⁺ 5.42 5.42 5.325.38 5.42 Al³⁺ 33.70 33.70 33.25 33.30 33.70 Mg²⁺ 6.83 6.83 7.70 6.635.83 Ca²⁺ 30.52 28.72 28.50 29.52 27.72 Sr²⁺ 17.16 17.16 17.16 17.0017.16 Ba²⁺ 2.91 4.70 4.60 4.70 5.70 Li⁺ 1.00 1.00 1.00 1.00 1.00 Na⁺1.20 1.20 1.20 1.20 1.20 K⁺ 0.00 0.00 0.00 0.00 1.00 Y³⁺ 1.27 1.27 1.271.27 1.27 Zn²⁺ 0.00 0.00 0.00 0.00 1.00 Anionic % O²⁻ 8.24 8.24 8.178.17 8.28 Cl⁻ 0.17 0.17 0.17 0.17 0.00 F⁻ 91.59 91.59 91.66 91.66 91.72O²⁻/P⁵⁺ 3.5 3.5 3.5 3.5 3.5 N_(d) 1.43062 1.43295 1.43128 1.432561.43252 ν_(d) 95.9 96 96.3 95.9 95.9 Tg (° C.) 415 421 418 417 LT (° C.)650 650 650 650 650 Nd(1 h) 1.43062 1.43295 1.43128 1.43256 1.43252 Nd(3h) 1.43415 Nd(3 h) − Nd(1 h) 0.0012 |Nd(3 h) − Nd(1 h)| 0.0012 (Notes)Nd(1 h) shows a refractive index (n_(d)) of a sample obtained by meltingat 900° C. for 1 hour. Nd(3 h) shows a refractive index (n_(d)) of asample obtained by melting at 900° C. for 3 hours. ν_(d) (1 h) shows anAbbe's number (ν_(d)) of a sample obtained by melting at 900° C. for 1hour. ν_(d) (3 h) shows an Abbe's number (ν_(d)) of a sample obtained bymelting at 900° C. for 3 hours.

TABLE 1-7 Optical glass No. 31 32 33 34 35 Cationic % P⁵⁺ 5.42 5.42 5.425.17 5.17 Al³⁺ 33.70 33.70 33.70 35.09 35.09 Mg²⁺ 6.83 5.83 6.83 4.074.07 Ca²⁺ 28.72 27.72 28.72 27.26 25.38 Sr²⁺ 17.16 18.16 16.16 15.0915.09 Ba²⁺ 4.70 5.70 4.70 4.52 9.52 Li⁺ 1.00 1.00 1.00 6.12 3.00 Na⁺1.20 1.20 1.20 0.00 0.00 K⁺ 0.00 0.00 0.00 0.00 0.00 Y³⁺ 1.27 1.27 1.272.68 2.68 Zn²⁺ 0.00 0.00 1.00 0.00 0.00 Anionic % O²⁻ 8.24 8.24 8.247.91 7.80 Cl⁻ 0.00 0.00 0.00 0.17 0.17 F⁻ 91.76 91.76 91.76 91.92 92.03O²⁻/P⁵⁺ 3.5 3.5 3.5 3.5 3.5 N_(d) 1.43229 1.4345 1.43224 1.43165 1.43795ν_(d) 96.9 95.9 96.1 95.7 95.4 Tg (° C.) 418 419 415 407 LT (° C.) 650650 650 650 650 Nd(1 h) 1.43229 1.4345 1.43224 1.43165 1.43795 Nd(3 h)1.43355 Nd(3 h) − Nd(1 h) 0.00190 |Nd(3 h) − Nd(1 h)| 0.00190 (Notes)Nd(1 h) shows a refractive index (n_(d)) of a sample obtained by meltingat 900° C. for 1 hour. Nd(3 h) shows a refractive index (n_(d)) of asample obtained by melting at 900° C. for 3 hours. ν_(d) (1 h) shows anAbbe's number (ν_(d)) of a sample obtained by melting at 900° C. for 1hour. ν_(d) (3 h) shows an Abbe's number (ν_(d)) of a sample obtained bymelting at 900° C. for 3 hours.

TABLE 1-8 Optical glass No. 36 37 38 Cationic % P⁵⁺ 5.17 5.17 4.67 Al³⁺36.09 35.09 35.59 Mg²⁺ 4.07 4.07 4.07 Ca²⁺ 25.38 25.38 23.26 Sr²⁺ 15.0915.09 15.09 Ba²⁺ 8.52 8.52 8.52 Li⁺ 3.00 3.00 6.12 Na⁺ 0.00 0.00 0.00 K⁺0.00 0.00 0.00 Y³⁺ 2.68 3.68 2.68 Zn²⁺ 0.00 0.00 0.00 Anionic % O²⁻ 7.777.77 7.12 Cl⁻ 0.17 0.17 0.17 F⁻ 92.06 92.06 92.71 O²⁻/P⁵⁺ 3.5 3.5 3.5N_(d) 1.43644 1.43811 1.43382 ν_(d) 95.7 95.7 95.8 Tg ( ) 410 409 390 LT( ) 650 650 670 Nd (1 h) 1.43644 1.43811 1.43382 Nd (3 h) 1.43493 Nd (3h) − Nd (1 h) 0.00111 |Nd (3 h) − Nd (1 h)| 0.00111 (Notes) Nd (1 h)shows a refractive index (n_(d)) of a sample obtained by melting at 900°C. for 1 hour. Nd (3 h) shows a refractive index (n_(d)) of a sampleobtained by melting at 900° C. for 3 hours. ν_(d) (1 h) shows an Abbe'snumber (ν_(d)) of a sample obtained by melting at 900° C. for 1 hour.ν_(d) (3 h) shows an Abbe's number (ν_(d)) of a sample obtained bymelting at 900° C. for 3 hours.

In the optical glass of the present invention, there is only a smalldifference in Abbe's number (ν_(d)) depending upon the time period formelting raw materials, and hence any one of ν_(d) (1 h) and ν_(d) (3 h)may be used as an Abbe's number (ν_(d)). When a rigorous Abbe's number(ν_(d)) is to be determined, ν_(d) (1 h) shall be used as the Abbe'snumber (ν_(d)) of the optical glass of the present invention.

In addition, each of the above optical glasses Nos. 1 to 38 may contain0.5 to 13 cationic %, based on the corresponding glass compositionexcluding Cu²⁺, of Cu²⁺ for use as a near infrared absorption glass.

Striae were not found in any one of the optical glasses Nos. 1 to 38 andoptical glasses obtained by incorporating 0.5 to 13 cationic %, based onthe corresponding glass composition excluding Cu²⁺, of Cu²⁺ to each ofthese optical glasses, and they were optically very homogeneous.

Example 2 Production Examples of Press-Molding Preforms

Preforms were produced from the optical glasses Nos. 1 to 38 shown inTables 1-1 to 1-8 in the following manner. A molten glass was caused toflow at a constant flow rate out of a pipe made of a platinum alloy thatwas temperature-adjusted to a temperature range in which the moltenglass could be caused to stably flow out without devitrification of theglass. And, molten glass gobs were separated by a method in which theglass gob was allowed to drop or a method in which the forward end ofthe molten glass was supported with a support and then the support wasrapidly moved down to separate the glass gob. It should be understoodthat each of the thus-obtained molten glass gobs had the weight obtainedby adding the weight of removal amount to the weight of one preformintended.

Then, the thus-obtained molten glass gobs were received with receivingmolds with an ejection port in the bottom portion thereof each, andpress-molding preforms were produced while the glass gobs were caused tofloat by ejecting a gas from the gas ejection ports. The preforms wereimparted with the form of a sphere or a flattened sphere by adjustingand setting the intervals of separation from the molten glass. Theweight each of the obtained preforms was accurately in agreement withset values, and all the preforms had smooth surfaces.

Further, as another method, the entire surface of each of shapedspherical preforms was polished by a known method to remove the entiresurface layer, whereby optically homogeneous preforms were obtained.

Glass plates were obtained from the optical glasses Nos. 1 to 38 shownin Tables 1-1 to 1-8 in the following manner. A molten glass was causedto flow at a constant flow rate out of a pipe made of a platinum alloythat was temperature-adjusted to a temperature range in which the moltenglass could be caused to stably flow out without devitrification of theglass. While the molten glass was continuously cast into a casting mold,a shaped glass plate was drawn through an opening portion on a side ofthe casting mold at a constant speed in the horizontal direction, andallowed to pass through the inside of an annealing furnace to remove astrain. Then, it was cut to a desired length to give glass plates one byone.

Then, each glass plate was cut into pieces in the form of dice toprepare a plurality of glass pieces, and these glass pieces were groundand polished to give optically homogeneous preforms having smoothsurfaces.

Example 3 Production Example of Optical Elements

Each of the above-obtained preforms was precision press-molded with apress apparatus shown in FIG. 1 to give aspherical lenses.

That is, there was provided a press mold having an upper mold member 1,a lower mold member 2 and a sleeve 3, and a preform was placed betweenthe lower mold member 2 and the upper mold member 1. Then, a nitrogenatmosphere was introduced into a quartz tube 11, and a heater 12 iselectrically powered to heat the inside of the quartz tube 11. Thetemperature inside the press mold was set at a temperature at which theglass to be molded exhibited a viscosity of 10⁸ to 10¹⁰ dPa·s, and whilethis temperature was maintained, a press rod 13 was moved downward topress the upper mold member 1, and the preform set in the mold wasthereby pressed. The pressure of the press was set at 8 MPa, and thepressing time period was set for 30 seconds. After the pressing, thepressure of the press was removed, and the glass molded product obtainedby the press molding was gradually cooled to a temperature at which theabove glass exhibited a viscosity of 10¹² dPa·s or more in state whereit was in contact with the lower mold member 2 and the upper mold member1. Then, it was rapidly cooled to room temperature and then the glassmolded product was taken out of the mold to give an aspherical lens. Thespherical lenses obtained in the above manner had very high surfaceaccuracy.

In FIG. 1, reference numeral 9 indicates a support rod, referencenumeral 10 indicates a lower mold member/sleeve holder, and referencenumeral 14 indicates a thermocouple.

Some of the aspherical lenses obtained by the precision press-moldingwere provided with an anti-reflection film each as required.

Preforms that were the same as the above preforms were precisionpress-molded by a method different from the above method. In thismethod, first, a preform was pre-heated up to a temperature at which theglass constituting the preform exhibited a viscosity of 10⁸ dPa·s whileit was caused to float. On the other hand, a press mold having an uppermold member, a lower mold member and a sleeve was heated up to atemperature at which the glass constituting the above preform exhibiteda viscosity of 10⁹ to 10¹² dPa·s, and the above pre-heated preform wasintroduced into the cavity of the press mold and precision press-moldedat 10 MPa. Upon the start of the pressing, the cooling of the glass andthe press mold was started, and they were cooled to a temperature atwhich the molded glass came to have a viscosity of 10¹² dPa·s or more.Then, the molded product was taken out of the mold to give an asphericallens. Aspherical lenses obtained in the above manner had very highsurface accuracy.

Some of the aspherical lenses obtained by the above precisionpress-molding were provided with an anti-reflection film each asrequired.

In the above manners, there were highly productively and highlyaccurately produced optical elements that had high inside quality eachand that were made of glass.

Example 4 Production Example of Optical Element Blank

Optical element blanks were produced from the optical glasses Nos. 1 to38 shown in Tables 1-1 to 1-8 in the following manner. A molten glasswas caused to flow at a constant flow rate out of a pipe made of aplatinum alloy that was temperature-adjusted to a temperature range inwhich the molten glass could be caused to stably flow out withoutdevitrification of the glass, and the molten glass was fed on themolding surface of a lower mold member constituting a press mold.Incidentally, prior to the feeding of the molten glass, a mold releaseagent in the form of a powder such as a boron nitride powder isuniformly applied to the molding surface of the lower mold member.

Then, the molten glass that is flowing out is cut with a cutting bladecalled shears, to obtain a molten glass gob on the molding surface ofthe lower mold member.

Then, the lower mold member with the molten glass gob on it isintroduced into a position where the upper mold member of the press moldis standing above, and the molten glass gob is pressed with the upperand lower mold members while the glass gob is in a softened state. Theupper and lower mold members were separated from each other, and apress-molded product obtained in this manner was released taken out ofthe mold to give an optical element blank. Then, the thus-obtained blankwas annealed to remove a strain and to adjust its optical propertiessuch that the optical properties such as a refractive index, etc., cameto be precisely equal to desired values, to give an optical elementblank having a desired from. In this manner, there were produced lensblanks having forms similar to those of various spherical lenses such asa convex meniscus lens, a concave meniscus lens, a plano-convex lens, aplano-concave lens, a biconvex lens, a biconcave lens, and the like.

Separately, optical element blanks were produced from the opticalglasses Nos. 1 to 38 shown in Tables 1-1 to 1-8 in the following manner.A molten glass was caused to flow at a constant flow rate out of a pipemade of a platinum alloy that was temperature-adjusted to a temperaturerange in which the molten glass could be caused to stably flow outwithout devitrification of the glass, and while the molten glass wascontinuously cast into a casting mold, a shaped glass plate waswithdrawn at a constant speed in the horizontal direction through anopening portion on a side of the casting mold and allowed to passthrough an inside of an annealing furnace to remove a strain. Then, itwas cut to a desired length to give glass plates one by one.

Then, the glass plates were cut to produce a plurality of glass piecesin the form of dice, and these glass pieces were barrel-ground to removeedges of the glass pieces and to adjust their weights so that they haddesired weights, whereby there were obtained preforms whose surfaceswere roughened.

Boron nitride in the form of a powder was uniformly applied to theentire surface of each preform, the preforms were placed on refractoryplates and the plates were placed in a heating furnace to heat andsoften the preforms. A softened preform was introduced into a press moldand press-molded. Optical element blanks were obtained in this manner.

The thus-obtained optical element blanks were annealed to remove astrain and to adjust their optical properties such that the opticalproperties such as a refractive index, etc., came to be precisely equalto desired values, to give an optical element blank having a desiredfrom. In this manner, there were produced lens blanks having formssimilar to those of various spherical lenses such as a convex meniscuslens, a concave meniscus lens, a plano-convex lens, a plano-concavelens, a biconvex lens, a biconcave lens, and the like.

Example 5 Production Example of Optical Element

The optical element blanks obtained in Example 4 were ground andpolished to give various spherical lenses such as a convex meniscuslens, a concave meniscus lens, a plano-convex lens, a plano-concavelens, a biconvex lens, a biconcave lens, and the like.

Further, the annealed glass plates obtained in Example 4 were cut andthe cut pieces were ground and polished to give various spherical lensessuch as a convex meniscus lens, a concave meniscus lens, a plano-convexlens, a plano-concave lens, a biconvex lens, a biconcave lens, and thelike.

In the above manners, there were highly productively and highlyaccurately produced optical elements that had high inside quality eachand that were made of glass.

Example 6 Production Example of Optical Element

Near infrared absorption glasses were obtained from the optical glassesNos. 1 to 38 obtained in Example 1 by adding 0.5 to 13 cationic % ofCu²⁺ based on the above optical glasses excluding Cu²⁺, and they weresliced to obtain flat plates. Main surfaces of the flat plates wereoptically polished to produce near infrared absorption filters.

INDUSTRIAL UTILITY

According to the present invention, there can be obtained alow-dispersion optical glass, which is capable of suppressing thevolatilization of a glass component and the variation of qualityinvolved in the fluctuations of a glass composition when an opticalglass formed of a fluorophosphate glass is produced or when the producedglass in a molten state is caused to flow out of a pipe and shaped intoa glass shaped material, and there can be produced press-moldingpreforms from the above optical glass and, further, optical elementssuch as various lenses, and the like.

The invention claimed is:
 1. An optical glass that has an Abbe's number(νd) of over 70 and that is formed of a fluorophosphate glass having anLi⁺ content of 0.5 to 30 cationic % and an O²⁻ content/P⁵⁺ content molarratio, O²⁻/P⁵⁺, of 3.50 or more.
 2. The optical glass of claim 1, whichis formed of a fluorophosphate glass having a refractive index (nd) ofover 1.53, having a total rare earth element of less than 5 cationic %and a B³⁺ content of 0 to 5 cationic %, and having an F⁻ content/F⁻ andO²⁻ total content molar ratio, F⁻/(F⁻+O²⁻), of 0.3 or more.
 3. Theoptical glass of claim 2, which contains no ion having absorption in thevisible light region.
 4. The optical glass of claim 2, wherein thefluorophosphate glass comprises, by cationic %, 20 to 50% of P⁵⁺, 5 to40% of Al³⁺, 0 to 10% of Mg²⁺, 0 to 20% of Ca²⁺, 0 to 20% of Sr²⁺, 0 to40% of Ba²⁺, provided that the total content of Mg²⁺, Ca²⁺, Sr²⁺ andBa²⁺ is 10% or more, 0.5 to 30% of Li⁺, 0 to 20% of Na⁺, 0 to 20% of K⁺,0 to 5% of Y³⁺, exclusive of 5%, 0 to 5% of La³⁺, exclusive of 5%, 0 to5% of Gd³⁺, exclusive of 5%, 0 to 5% of Yb³⁺, exclusive of 5%, providedthat the total content of Y³⁺, La³⁺, Gd³⁺ and Yb³⁺ is less than 5%, 0 to5% of B³⁺, 0 to 20% of Zn²⁺ and 0 to 20% of In³⁺.
 5. The optical glassof claim 2, wherein the fluorophosphate glass comprises, by cationic %,20 to 50% of P⁵⁺, 5 to 40% of Al³⁺, 0 to 10% of Mg²⁺, 0 to 20% of Ca²⁺,0 to 20% of Sr²⁺, 0 to 40% of Ba²⁺, provided that the total content ofMg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ is 10% or more, 0.5 to 30% of Li⁺, 0 to 20% ofNa⁺, 0 to 20% of K⁺, 0 to 5% of Y³⁺, exclusive of 5%, 0 to 5% of La³⁺,exclusive of 5%, 0 to 5% of Gd³⁺, exclusive of 5%, 0 to 5% of Yb³⁺,exclusive of 5%, provided that the total content of Y³⁺, La³⁺, Gd³⁺ andYb³⁺ is less than 5%, 0 to 5% of B³⁺, 0 to 20% of Zn²⁺ and 0 to 20% ofIn³⁺.
 6. The optical glass of claim 4, which contains no ion havingabsorption in the visible light region.
 7. A process for the productionof an optical glass formed of a fluorophosphate glass, which comprisesusing, as raw materials or cullet, a glass composition having a totalO²⁻ content/total P⁵⁺ content molar ratio, O²⁻/P⁵⁺, of 3.50 or more whenan optical glass is produced by melting the raw materials or cullet andrefining and homogenizing a molten glass, and thereby producing theoptical glass recited in claim
 1. 8. A process for the production of anoptical glass formed of a fluorophosphate glass, which comprisespreparing a raw material batch from raw materials or cullet, melting theraw material batch and then carrying out refining and homogenization,the process comprising preparing the raw material batch in which the Li⁺content is 0.5 to 30 cationic % and the total O²⁻ content/total P⁵⁺content molar ratio, O²⁻/P⁵⁺, is 3.5 or more and carrying out themelting, refining and homogenization to produce a fluorophosphate glasshaving an Abbe's number (ν_(d)) of over
 70. 9. A process for theproduction of an optical glass formed of a fluorophosphate glass, whichcomprises preparing a raw material batch from raw materials or cullet,melting the raw material batch, then carrying out refining andhomogenization to prepare a molten glass having an Li⁺ content of 0.5 to30 cationic %, an O²⁻ content/P⁵⁺ content molar ratio of, O²⁻/P⁵⁺, of3.5 or more, and an Abbe's number (νd) of over 70, and shaping saidmolten glass, the process comprising controlling the total O²⁻content/total P⁵⁺ content molar ratio, O²⁻/P⁵⁺, in said raw materialbatch for decreasing the volatility of said molten glass.
 10. Theprocess for the production of an optical glass as recited in claim 8,wherein the fluorophosphate glass having a rare earth element totalcontent of less than 5 cationic %, an F⁻ content/F⁻ and O²⁻ totalcontent molar ratio, F⁻/(F⁻+O²⁻), of 0.3 or more and a refractive index(n_(d)) of over 1.53 is produced.
 11. The process for the production ofan optical glass as recited in claim 10, wherein the fluorophosphateglass containing no ion having absorption in the visible light region isproduced.
 12. An optical element formed of the optical glass recited inclaim
 1. 13. A process for the production of an optical glass formed ofa fluorophosphate glass, which comprises preparing a raw material batchfrom raw materials or cullet, melting the raw material batch and thencarrying out refining and homogenization, the process comprisingpreparing the raw material batch in which Li⁺ content is 0.5 to 30cationic % and the total O²⁻ content/total P⁵⁺ content molar ratio,O²⁻/P⁵⁺, is 3.5 or more and carrying out the melting, refining andhomogenization to produce a fluorophosphate glass having no ion havingabsorption in the visible light region, a rare earth element totalcontent of less than 5 cationic %, an F⁻ content/F⁻ and O²⁻ totalcontent molar ratio, F⁻/(F⁻+O²⁻), of 0.3 or more, an Abbe's number(ν_(d)) of over 70 and a refractive index (n_(d)) of over 1.53.